Means and method for the detection of cardiac events

ABSTRACT

Disclosed is a system for the detection of cardiac events that includes an implanted device called a cardiosaver, a physician&#39;s programmer and an external alarm system. The system is designed to provide early detection of cardiac events such as acute myocardial infarction or exercise induced myocardial ischemia caused by an increased heart rate or exertion. The system can also alert the patient with a less urgent alarm if a heart arrhythmia is detected. Using different algorithms, the cardiosaver can detect a change in the patient&#39;s electrogram that is indicative of a cardiac event within five minutes after it occurs and then automatically warn the patient that the event is occurring. To provide this warning, the system includes an internal alarm sub-system (internal alarm means) within the cardiosaver and/or an external alarm system (external alarm means) which are activated after the ST segment of the electrogram exceeds a preset threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/642,245 which is a continuation-in-partapplication of the now issued U.S. Pat. No. 6,609,023.

FIELD OF USE

[0002] This invention is in the field of systems, including devicesimplanted within a human patient, for the purpose of automaticallydetecting the onset of a cardiac event.

BACKGROUND OF THE INVENTION

[0003] Heart disease is the leading cause of death in the United States.A heart attack (also known as an Acute Myocardial Infarction (AMI))typically results from a thrombus that obstructs blood flow in one ormore coronary arteries. AMI is a common and life-threateningcomplication of coronary heart disease. The sooner that perfusion of themyocardium is restored (e.g., with injection of a thrombolyticmedication such as tissue plasminogen activator (tPA)), the better theprognosis and survival of the patient from the heart attack. The extentof damage to the myocardium is strongly dependent upon the length oftime prior to restoration of blood flow to the heart muscle.

[0004] Myocardial ischemia is caused by a temporary imbalance of blood(oxygen) supply and demand in the heart muscle. It is typically provokedby physical activity or other causes of increased heart rate when one ormore of the coronary arteries are obstructed by atherosclerosis.Patients will often (but not always) experience chest discomfort(angina) when the heart muscle is experiencing ischemia.

[0005] Acute myocardial infarction and ischemia may be detected from apatient's electrocardiogram (ECG) by noting an ST segment shift (i.e.,voltage change) over a relatively short (less than 5 minutes) period oftime. However, without knowing the patient's normal ECG patterndetection from standard 12 lead ECG can be unreliable. In addition,ideal placement of subcutaneous electrodes for detection of ST segmentshifts as they would relate to a subcutaneously implanted device has notbeen explored in the prior art.

[0006] Fischell et al in U.S. Pat. Nos. 6,112,116 and 6,272,379 describeimplantable systems for detecting the onset of acute myocardialinfarction and providing both treatment and alarming to the patient.While Fischell et al discuss the detection of a shift in the S-T segmentof the patient's electrogram from an electrode within the heart as thetrigger for alarms; it may be desirable to provide more sophisticateddetection algorithms to reduce the probability of false positive andfalse negative detection. In addition while these patents describe somedesirable aspects of programming such systems, it may be desirable toprovide additional programmability and alarm control features.

[0007] Although anti-tachycardia pacemakers and Implantable CardiacDefibrillators (ICDs) can detect heart arrhythmias, none are currentlydesigned to detect ischemia and acute myocardial infarction eventsindependently or in conjunction with arrhythmias.

[0008] In U. S. Pat. Nos. 6,112,116 and 6,272,379 Fischell et al,discuss the storage of recorded electrogram and/or electrocardiogramdata; however techniques to optimally store the appropriate electrogramand/or electrocardiogram data and other appropriate data in a limitedamount of system memory are not detailed.

[0009] In U. S. Pat. No. 5,497,780 by M. Zehender, a device is describedthat has a “goal of eliminating . . . cardiac rhythm abnormality.” To dothis, Zehender requires exactly two electrodes placed within the heartand exactly one electrode placed outside the heart. Although multipleelectrodes could be used, the most practical sensor for providing anelectrogram to detect a heart attack would use a single electrode placedwithin or near to the heart.

[0010] Zehender's drawing of the algorithm consists of a single boxlabeled ST SIGNAL ANALYSIS with no details of what the analysiscomprises. His only description of his detection algorithm is to use acomparison of the ECG to a reference signal of a normal ECG curve.Zehender does not discuss any details to teach an algorithm by whichsuch a comparison can be made, nor does Zehender explain how oneidentifies the “normal ECG curve”. Each patient will likely have adifferent “normal” baseline ECG that will be an essential part of anysystem or algorithm for detection of a heart attack or ischemia.

[0011] In addition, Zehender suggests that an ST signal analysis shouldbe carried out every three minutes. It may be desirable to use bothlonger and shorter time intervals than 3 minutes so as to capturecertain changes in ECG that are seen early on or later on in theevolution of an acute myocardial infarction. Longer observation periodswill also be important to account for minor slowly evolving changes inthe “baseline” ECG. Zehender has no mention of detection of ischemiahaving different normal curves based on heart rate. To differentiatefrom exercise induced ischemia and acute myocardial infarction, it maybe important to correlate ST segment shifts with heart rate or R-Rinterval.

[0012] Finally, Zehender teaches that “if an insufficient blood supplyin comparison to the reference signal occurs, the corresponding abnormalST segments can be stored in the memory in digital form or as anumerical event in order to be available for associated telemetry at anytime.” Storing only abnormal ECG segments may miss important changes inbaseline ECG. Thus it is desirable to store some historical ECG segmentsin memory even if they are not “abnormal”.

[0013] The Reveal™ subcutaneous loop Holter monitor sold by Medtronicuses two case electrodes spaced by about 3 inches to recordelectrocardiogram information looking for arrhythmias. It has no realcapability to detect ST segment shift and its high pass filtering wouldin fact preclude accurate detection of changes in the low frequencyaspects of the heart's electrical signal. Also the spacing of theelectrodes it too close together to be able to effectively detect andrecord ST segment shifts. Similarly, current external Holter monitorsare primarily designed for capturing arrhythmia related signals from theheart.

[0014] Although often described as an electrocardiogram (ECG), thestored electrical signal from the heart as measured from electrodeswithin the body should be termed an “electrogram”. The early detectionof an acute myocardial infarction or exercise induced myocardialischemia caused by an increased heart rate or exertion is feasible usinga system that notes a change in a patient's electrogram. The portion ofsuch a system that includes the means to detect a cardiac event isdefined herein as a “cardiosaver” and the entire system including thecardiosaver and the external portions of the system is defined herein asa “guardian system.”

[0015] Furthermore, although the masculine pronouns “he” and “his” areused herein, it should be understood that the patient or the medicalpractitioner who treats the patient could be a man or a woman. Stillfurther the term; “medical practitioner” shall be used herein to meanany person who might be involved in the medical treatment of a patient.Such a medical practitioner would include, but is not limited to, amedical doctor (e.g., a general practice physician, an internist or acardiologist), a medical technician, a paramedic, a nurse or anelectrogram analyst. A “cardiac event” includes an acute myocardialinfarction, ischemia caused by effort (such as exercise) and/or anelevated heart rate, bradycardia, tachycardia or an arrhythmia such asatrial fibrillation, atrial flutter, ventricular fibrillation, andpremature ventricular or atrial contractions (PVCs or PACs).

[0016] For the purpose of this invention, the term “electrocardiogram”is defined to be the heart electrical signals from one or more skinsurface electrode(s) that are placed in a position to indicate theheart's electrical activity (depolarization and repolarization). Anelectrocardiogram segment refers to the recording of electrocardiogramdata for either a specific length of time, such as 10 seconds, or aspecific number of heart beats, such as 10 beats. For the purposes ofthis specification the PQ segment of a patient's electrocardiogram isthe typically flat segment of a beat of an electrocardiogram that occursjust before the R wave.

[0017] For the purpose of this invention, the term “electrogram” isdefined to be the heart electrical signals from one or more implantedelectrode(s) that are placed in a position to indicate the heart'selectrical activity (depolarization and repolarization). An electrogramsegment refers to the recording of electrogram data for either aspecific length of time, such as 10 seconds, or a specific number ofheart beats, such as 10 beats. For the purposes of this specificationthe PQ segment of a patient's electrogram is the typically flat segmentof an electrogram that occurs just before the R wave. For the purposesof this specification, the terms “detection” and “identification” of acardiac event have the same meaning. A beat is defined as a sub-segmentof an electrogram or electrocardiogram segment containing exactly one Rwave.

[0018] Heart signal parameters are defined to be any measured orcalculated value created during the processing of one or more beats ofelectrogram data. Heart signal parameters include PQ segment averagevalue, ST segment average voltage value, R wave peak value, STdeviation, ST shift, average signal strength, T wave peak height, T waveaverage value, T wave deviation, heart rate, R-R interval andpeak-to-peak voltage amplitude.

SUMMARY OF THE INVENTION

[0019] The present invention is a system for the detection of cardiacevents (a guardian system) that includes a device called a cardiosaver,and external equipment including a physician's programmer and anexternal alarm system. The present invention envisions a system forearly detection of an acute myocardial infarction or exercise inducedmyocardial ischemia caused by an increased heart rate or exertion.

[0020] In the preferred embodiment of the present invention, thecardiosaver is implanted along with the electrodes. In an alternateembodiment, the cardiosaver and the electrodes could be external butattached to the patient's body. Although the following descriptions ofthe present invention in most cases refer to the preferred embodiment ofan implanted cardiosaver processing electrogram data from implantedelectrodes, the techniques described are equally applicable to thealternate embodiment where the external cardiosaver processeselectrocardiogram data from skin surface electrodes.

[0021] In the preferred embodiment of the cardiosaver either or bothsubcutaneous electrodes or electrodes located on a pacemaker type rightventricular or atrial leads will be used. It is also envisioned that oneor more electrodes may be placed within the superior vena cava. Oneversion of the implanted cardiosaver device using subcutaneouselectrodes would have an electrode located under the skin on thepatient's left side. This could be best located between 2 and 20 inchesbelow the patient's left arm pit. The cardiosaver case that would act asthe indifferent electrode would typically be implanted like a pacemakerunder the skin on the left side of the patient's chest.

[0022] Using one or more detection algorithms, the cardiosaver candetect a change in the patient's electrogram that is indicative of acardiac event, such as an acute myocardial infarction, within fiveminutes after it occurs and then automatically warn the patient that theevent is occurring. To provide this warning, the guardian systemincludes an internal alarm sub-system (internal alarm means) within thecardiosaver and/or an external alarm system (external alarm means). Inthe preferred, implanted embodiment, the cardiosaver communicates withthe external alarm system using a wireless radio-frequency (RF) signal.

[0023] The internal alarm means generates an internal alarm signal towarn the patient. The internal alarm signal may be a mechanicalvibration, a sound or a subcutaneous electrical tickle. The externalalarm system (external alarm means) will generate an external alarmsignal to warn the patient. The external alarm signal is typically asound that can be used alone or in combination with the internal alarmsignal. The internal or external alarm signals would be used to alertthe patient to at least two different types of conditions (i.e. levelsof severity): an “EMERGENCY ALARM” signaling the detection of a majorcardiac event (e.g. a heart attack) and the need for immediate medicalattention, and a less critical “SEE DOCTOR ALERT” (or alarm) signalingthe detection of a less serious non life threatening condition such asexercise induced ischemia. The SEE DOCTOR alert signal would be used totell the patient that he is not in immediate danger but should arrangean appointment with his doctor in the near future. In addition to thesignaling of less critical cardiac events, the SEE DOCTOR alert signalcould also signal the patient when the cardiosaver battery is gettinglow.

[0024] In the preferred embodiment, the internal EMERGENCY alarm signalwould be applied periodically, for example, with three pulses every 5seconds after the detection of a major cardiac event. It is alsoenvisioned that the less critical SEE DOCTOR alert, would be signaled ina different way, such as one pulse every 7 seconds.

[0025] The external alarm system is a hand-held portable device that mayinclude any or all of the following features:

[0026] 1. an external alarm means to generate an external alarm signalto alert the patient.

[0027] 2. the capability to receive cardiac event alarms, recordedelectrogram and other data from the cardiosaver

[0028] 3. the capability to transmit the cardiac event alarm, recordedelectrogram and other data collected by the cardiosaver to a medicalpractitioner at a remote location.

[0029] 4. an “alarm-off” or disable button that when depressed canacknowledge that the patient is aware of the alarm and will turn offinternal and external alarm signals.

[0030] 5. a display (typically an LCD panel) to provide informationand/or instructions to the patient by a text message and the display ofsegments of the patient's electrogram.

[0031] 6. the ability to provide messages including instructions to thepatient via a pre-recorded human voice.

[0032] 7. a patient initiated electrogram capture initiated by a “PanicButton” to allow the patient, even when there has been no alarm, toinitiate transmission of electrogram data from the cardiosaver to theexternal alarm system for transmission to a medical practitioner.

[0033] 8. a patient initiated electrogram capture to initiatetransmission of electrogram data from the cardiosaver to the externalalarm system for display to a medical practitioner using the display onthe external alarm system.

[0034] 9. the capability to automatically turn the internal and externalalarms off after a reasonable (initial alarm-on) period that istypically less than 30 minutes if the alarm-off button is not used. Thisfeature might also be implemented within the cardiosaver implant.

[0035] If the alarm disable button is not used by the patient toindicate acknowledgement of awareness of an EMERGENCY alarm, it isenvisioned that instead of completely stopping all alarm signals to thepatient after the first period of time which is an initial alarm-onperiod, a reminder alarm signal would be turned on for a second timeperiod which is a reminder alarm on-period of time that would follow anoff-period of time during which time the alarm signal is turned off.

[0036] The reminder alarm signal might be repeated periodically for athird longer time period which is a periodic reminder time period. Eachof the repeated reminder alarm signals would last for the reminder alarmon-period and would be followed by an alarm off-period. The periodicreminder time period would typically be 3 to 5 hours because after threeto five hours the patient's advantage in being alerted to seek medicalattention for a severe cardiac event like an AMI is mostly lost. Thealarm off-period between the periodic reminder alarm signals couldeither remain constant, increase or decrease over the periodic remindertime period. For example, after an initial alarm-on time period of fiveminutes a 30 second long reminder alarm signal might occur every 10minutes for a periodic reminder time period of 3 hours, (i.e. thereminder alarm on-period is 30 seconds and the alarm off-period is 9minutes and 30 seconds). It is also envisioned that the alarm off-periodmight change during the periodic reminder time period. For example, theoff-period in the first hour of the periodic reminder time period mightbe 10 minutes increasing to 20 minutes in the last hour of the periodicreminder time period.

[0037] Text and/or spoken instructions may include a message that thepatient should promptly take some predetermined medication such aschewing an aspirin, placing a nitroglycerine tablet under his tongue,inhaling or nasal spraying a single or multiple drug combination and/orinjecting thrombolytic drugs into a subcutaneous drug port. Themessaging displayed by or spoken from the external alarm system and/or aphone call from a medical practitioner who receives the alarm could alsoinform the patient that he should wait for the arrival of emergencymedical services or he should promptly proceed to an emergency medicalfacility. It is envisioned that the external alarm system can havedirect connection to a telephone line and/or work through cell phone orother wireless networks.

[0038] If a patient seeks care in an emergency room, the external alarmsystem could provide a display to the medical practitioners in theemergency room of both the electrogram segment that caused the alarm andthe baseline electrogram segment against which the electrogram thatcaused the alarm was compared. The ability to display both baseline andalarm electrogram segments will significantly improve the ability of theemergency room physician to properly identify AMI.

[0039] A preferred embodiment of the external alarm system consists ofan external alarm transceiver and a handheld computer. The externalalarm transceiver having a standardized interface, such as Compact Flashadapter interface, a secure digital (SD) card interface, a multi-mediacard interface, a memory stick interface or a PCMCIA card interface. Thestandardized interface will allow the external alarm transceiver toconnect into a similar standardized interface slot that is present inmany handheld computers such as a Palm Pilot or Pocket PC. An advantageof this embodiment is that the handheld computer can cost effectivelysupply the capability for text and graphics display and for playingspoken messages.

[0040] Using a handheld computer, such as the Thera™ by Audiovox™ thatcombines a Pocket PC with having an SD/Multimedia interface slot with acell phone having wireless internet access, is a solution that caneasily be programmed to provide communication between the external alarmsystem and a diagnostic center staffed with medical practitioners.

[0041] The panic button feature, which allows a patient-initiatedelectrogram capture and transmission to a medical practitioner, willprovide the patient with a sense of security knowing that, if he detectssymptoms of a heart-related ailment such as left arm pain, chest pain orpalpitations, he can get a fast review of his electrogram. Such a reviewwould allow the diagnosis of arrhythmias, such as premature atrial orventricular beats, atrial fibrillation, atrial flutter or other heartrhythm irregularities. The medical practitioner could then advise thepatient what action, if any, should be taken. The guardian system wouldalso be programmed to send an alarm in the case of ventricularfibrillation so that a caretaker of the patient could be informed toimmediately provide a defibrillation electrical stimulus. This ispractical as home defibrillation units are now commercially available.It is also possible that, in patients prone to ventricular fibrillationfollowing a myocardial infarction, such a home defibrillator could beplaced on the patient's chest to allow rapid defibrillation shouldventricular fibrillation occur while waiting for the emergency medicalservices to arrive.

[0042] The physician's programmer provides the patient's doctor with thecapability to set cardiosaver cardiac event detection parameters. Theprogrammer communicates with the cardiosaver using the wirelesscommunication capability that also allows the external alarm system tocommunicate with the cardiosaver. The programmer can also be used toupload and review electrogram data captured by the cardiosaver includingelectrogram segments captured before, during and after a cardiac event.

[0043] An extremely important capability of the present invention is theuse of a continuously adapting cardiac event detection program thatcompares extracted features from a recently captured electrogram segmentwith the same features extracted from a baseline electrogram segment ata predetermined time in the past. For example, the thresholds fordetecting an excessive ST shift would be appropriately adjusted toaccount for slow changes in electrode sensitivity or ST segment voltagelevels over time. It may also be desirable to choose the predeterminedtime in the past for comparison to take into account daily cycles in thepatient's heart electrical signals. Thus, a preferred embodiment of thepresent invention would use a baseline for comparison that is collectedapproximately 24 hours prior to the electrogram segment being examined.Such a system would adapt to both minor (benign) slow changes in thepatient's baseline electrogram as well as any daily cycle.

[0044] Use of a system that adapts to slowly changing baselineconditions is of great importance in the time following the implantationof electrode leads in the heart. This is because there can be asignificant “injury current” present just after implantation of anelectrode and for a time of up to a month, as the implanted electrodeheals into the wall of the heart. Such an injury current may produce adepressed ST segment that deviates from a normal isoelectric electrogramwhere the PQ and ST segments are at approximately the same voltage.Although the ST segment may be depressed due to this injury current, theoccurrence of an acute myocardial infarction can still be detected sincean acute myocardial infarction will still cause a significant shift fromthis “injury current” ST baseline electrogram. Alternately, the presentinvention might be implanted and the detector could be turned on afterhealing of the electrodes into the wall of the heart. This healing wouldbe noted in most cases by the evolution to an isoelectric electrogram(i.e., PQ and ST segments with approximately the same voltages).

[0045] The present invention's ST detection technique involves recordingand processing baseline electrogram segments to calculate the thresholdfor myocardial infarction and/or ischemia detection. These baselineelectrogram segments would typically be collected, processed and storedonce an hour or with any other appropriate time interval.

[0046] A preferred embodiment of the present invention would save andprocess a 10 second baseline electrogram segment once every hour. Every30 seconds the cardiosaver would save and process a 10 second longrecent electrogram segment. The cardiosaver would compare the recentelectrogram segment with the baseline electrogram segment fromapproximately 24 hours before (i.e. 24±½ hour before).

[0047] The processing of each of the hourly baseline electrogramsegments would involve calculating the average electrogram signalstrength as well as calculating the average “ST deviation”. The STdeviation for a single beat of an electrogram segment is defined to bethe difference between the average ST segment voltage and the average PQsegment voltage. The average ST deviation of the baseline electrogramsegment is the average of the ST deviation of multiple (at least two)beats within the baseline electrogram segment.

[0048] The following detailed description of the drawings fullydescribes how the ST and PQ segments are measured and averaged.

[0049] An important aspect of the present invention is the capability toadjust the location in time and duration of the ST and PQ segments usedfor the calculation of ST shifts. The present invention is initiallyprogrammed with the time interval between peak of the R wave of a beatand the start of the PQ and ST segments of that beat set for thepatient's normal heart rate. As the patient's heart rate changes duringdaily activities, the present invention will adjust these time intervalsfor each beat proportional to the R-R interval for that beat. In otherwords, if the R-R interval shortens (higher heart rate) then the ST andPQ segments would move closer to the R wave peak and would becomeshorter. ST and PQ segments of a beat within an electrogram segment aredefined herein as sub-segments of the electrogram segment. Specifically,the time interval between the R wave and the start of the ST and PQsegments may be adjusted in proportion to the R-R interval oralternately by the square root of the R-R interval. It is preferable inall cases to base these times on the R-R interval from the beat beforethe current beat. As calculating the square root is a processorintensive calculation, the preferred implementation of this feature isbest done by pre-calculating the values for the start of PQ and STsegments during programming and loading these times into a simple lookuptable where for each R-R interval, the start times and/or durations forthe segments is stored.

[0050] It is envisioned that a combination of linear and square roottechniques could be used where both the time interval between the R waveand the start of the ST segment (T_(ST)) and the duration of the STsegment (D_(ST)) are proportional to the square root of the R-Rinterval, while the time interval between the R wave and the start ofthe PQ segment (T_(PQ)) and the duration of the PQ segment (D_(PQ)) arelinearly proportional to the R-R interval.

[0051] It is also envisioned that the patient would undergo a stresstest following implant, the electrogram data collected would betransmitted to the physician's programmer and the parameters T_(ST),D_(ST), T_(PQ) and D_(PQ) would be automatically selected by theprogrammer based on the electrogram data from the stress test. The datafrom the stress test would cover each of the heart rate ranges and couldalso be used by the programmer to generate excessive ST shift detectionthresholds for each of the heart rate ranges. In each heart rate rangeof the implant the detection threshold would typically be set based onthe mean and standard deviation of the ST shifts seen during the stresstest. For example, one could set the detection threshold for each heartrate range to the value of the mean ST shift plus or minus a multiple(e.g. three) times the standard deviation. In each case where theprogrammer can automatically select parameters for the ST shiftdetection algorithm, a manual override would also be available to themedical practitioner. Such an override is of particular importance as itallows adjustment of the algorithm parameters to compensate for missedevents or false positive detections.

[0052] The difference between the ST deviation on any single beat in arecently collected electrogram segment and a baseline average STdeviation extracted from a baseline electrogram segment is definedherein as the “ST shift” for that beat. The present invention envisionsthat detection of acute myocardial infarction and/or ischemia would bebased on comparing the ST shift of one or more beats with apredetermined detection threshold “H_(ST)”.

[0053] In U.S. application Ser. No. 10051743 that is incorporated hereinby reference, Fischell describes a fixed threshold for detection that isprogrammed by the patient's doctor. The present invention envisions thatthe threshold should rather be based on some percentage “P_(ST)” of theaverage signal strength extracted from the baseline electrogram segmentwhere P_(ST) is a programmable parameter of the cardiosaver device. The“signal strength” can be measured as peak-to-peak signal voltage, RMSsignal voltage or as some other indication of signal strength such asthe difference between the average PQ segment amplitude and the peak Rwave amplitude.

[0054] Similarly, it is envisioned that the value of P_(ST) might beadjusted as a function of heart rate so that a higher threshold could beused if the heart rate is elevated, so as to not trigger on exercisethat in some patients will cause minor ST segment shifts when there isnot a heart attack occurring. Alternately, lower thresholds might beused with higher heart rates to enhance sensitivity to detectexercise-induced ischemia. One embodiment of the present invention has atable stored in memory where values of P_(ST) for a preset number ofheart rate ranges, (e.g. 50-80, 81-90, 91-100, 101-120, 121-140) mightbe stored for use by the cardiosaver detection algorithm in determiningif an acute myocardial infarction or exercise induced ischemia ispresent.

[0055] Thus it is envisioned that the present invention would use thebaseline electrogram segments in 3 ways.

[0056] 1. To calculate a baseline average value of a feature such as STsegment voltage or ST deviation that is then subtracted from the valueof the same feature in recently captured electrogram segments tocalculate the shift in the value of that feature. E.g. the baselineaverage ST deviation is subtracted from the amplitude of the STdeviation on each beat in a recently captured electrogram segment toyield the ST shift for that beat.

[0057] 2. To provide an average signal strength used in calculating thethreshold for detection of a cardiac event. This will improve detectionby compensating for slow changes in electrogram signal strength overrelatively long periods of time.

[0058] 3. To provide a medical practitioner with information that willfacilitate diagnosis of the patient's condition. For example, thebaseline electrogram segment may be transmitted to a remotely locatedmedical practitioner and/or displayed directly to a medical practitionerin the emergency room.

[0059] For the purposes of the present invention, the term adaptivedetection algorithm is hereby defined as a detection algorithm for acardiac event where at least one detection-related threshold adapts overtime so as to compensate for relatively slow (longer than an hour)changes in the patient's normal electrogram.

[0060] The present invention might also include an accelerometer builtinto the cardiosaver where the accelerometer is an activity sensor usedto discriminate between elevated heart rate resulting from patientactivity as compared to other causes.

[0061] It is also envisioned that the present invention could havespecific programming to identify a very low heart rate (bradycardia) ora very high heart rate (tachycardia or fibrillation). While a very lowheart rate is usually not of immediate danger to the patient, itspersistence could indicate the need for a pacemaker. As a result, thepresent invention could use the “SEE DOCTOR” alert along with anoptional message sent to the external alarm system to alert the patientthat his heart rate is too low and that he should see his doctor as soonas convenient. On the other hand, a very high heart rate can signalimmediate danger thus it would be desirable to initiate an EMERGENCY ina manner similar to that of acute myocardial infarction detection. Whatis more, detections of excessive ST shift during high heart rates may bedifficult and if the high heart rate is the result of a heart attackthen it is envisioned that the programming of the present inventionwould use a major event counter that would turn on the alarm if thedevice detects a combination of excessive ST shift and overly high heartrate.

[0062] Another early indication of acute myocardial infarction is arapid change in the morphology of the T wave. Unfortunately, there aremany non-AMI causes of changes in the morphology of a T wave. However,these changes typically occur slowly while the changes from an AMI occurrapidly. Therefore one embodiment of this invention uses detection of achange in the T wave as compared to a baseline collected a short time(less than 30 minutes) in the past. The best embodiment is probablyusing a baseline collected between 1 and 5 minutes in the past. Such a Twave detector could look at the amplitude of the peak of the T wave. Analternate embodiment of the T wave detector might look at the averagevalue of the entire T wave as compared to the baseline. The thresholdfor T wave shift detection, like that of ST shift detection, can be apercentage P_(T) of the average signal strength of the baselineelectrogram segment. P_(T) could differ from P_(ST) if both detectorsare used simultaneously by the cardiosaver.

[0063] In its simplest form, the “guardian system” includes only thecardiosaver and a physician's programmer. Although the cardiosaver couldfunction without an external alarm system where the internal alarmsignal stays on for a preset period of time, the external alarm systemis highly desirable. One reason it is desirable is the button on theexternal alarm system that provides the means for of turning off thealarm in either or both the implanted device (cardiosaver) and theexternal alarm system. Another very important function of the externalalarm system is to facilitate display of both the baseline and alarmelectrogram segments to a treating physician to facilitate rapiddiagnosis and treatment for the patient.

[0064] As an implantable device, the present invention cardiosaver mustconserve power to allow a reasonable lifetime in a cosmeticallyacceptable package size. In U.S. Pat. No. 6,609,023, Fischell et aldescribe how the cardiosaver collects and processes electrogram data fora first predetermined, “segment time period” (e.g. 10 seconds) to lookfor a cardiac event and then going to a lower power usage sleep statefor a second predetermined “sleep state time period” (e.g. 20 seconds).Although it is desirable to look for cardiac events every 30 seconds asdescribed by Fischell et al, it is possible to decrease the use ofelectrical power by extending the time duration of the sleep state timeperiod to be greater than 20 seconds. Extending implant lifetime bydecreasing electrical power usage can be accomplished by utilizing alonger time duration for the sleep state time period to be (for example)on the order of 50 to 80 seconds.

[0065] While a 50 to 80 sleep state time period with a 10 second timeduration for the segment time period of data collection would increasethe life of the implant, the total cycle times of 60 to 90 seconds is acomparatively long time to wait if cardiac events are to be quicklydetected. The present invention cardiosaver utilizes an adaptive cycletime where the sleep state time period following detection of an“abnormal” electrogram segment is shorter than the sleep state timeperiod following detection of an electrogram segment that has nodetected abnormality. For example, the sleep state time period could be80 seconds following an electrogram segment where no abnormality isdetected and 20 seconds following an electrogram segment where anyabnormality (e.g. excessive ST shift or arrhythmia) is detected. In thisway, the function during any irregularity of heart signal would be thesame as the Fischell et al. cardiosaver, yet significant power savingswould be created during normal functioning of the heart.

[0066] It is also envisioned that the sleep state time period could beeven more adaptive so that the length of the sleep state time might berelated to the number of successive normal (no abnormality detected)electrogram segments. For example, one normal segment would be followedby a sleep state time period of 40 seconds, two normal segments by 50seconds, 3 normal segments by 80 seconds, and 4 or more normal segmentsby 110 seconds. There would typically be a maximum sleep state timeperiod used during long periods when all electrogram segments are normaland a minimum sleep time period that would be used following anydetected abnormality. The maximum and minimum sleep times could bepreset or programmable.

[0067] An abnormal electrogram segment is an electrogram segment whereone or more heart signal parameters extracted during the processing ofthe electrogram segment by the cardiosaver meets the criteria for anabnormal electrogram segment. The criteria for an abnormal electrogramsegment can be the same criteria used for detecting a cardiac eventwithin the electrogram segment. It is also envisioned that the criteriafor detecting an abnormal electrogram segment could be less stringentthat the criteria for detecting a cardiac event. For example, anabnormal segment might be detected using a threshold lower by a presetpercentage (e.g. 50%) than the respective threshold for the indicationof a cardiac event. In this way, the time to detection of the eventmight be reduced by getting to the shorter sleep time more quickly.

[0068] It is also highly desirable for the present invention guardiansystem to allow real time or near real time display of electrogram datafor diagnostic purposes. Such a display could be of great value in anemergency setting where fast review of the patient's current heartsignal is important. In a real time mode, the cardiosaver 5 of FIG. 1would simultaneously collect and transmit electrogram data to theexternal equipment 7 of FIG. 1.

[0069] In the near real time mode, the cardiosaver 5 would collect anelectrogram segment, process the electrogram segment looking forabnormalities and then transmit the segment to the external equipment 7.A typical cycle time for the near real time mode would be 15 secondsincluding 10 seconds for electrogram segment collection, 1 second forprocessing and 4 seconds for transmission to the external equipment 7.The results of the processing might also be transmitted along with thesegment.

[0070] Thus it is an object of this invention is to have a cardiosaverdesigned to detect the occurrence of a cardiac event by comparingbaseline electrogram data from a first predetermined time with recentelectrogram data from a second predetermined time.

[0071] Another object of the present invention is to have a Guardiansystem where the electrogram data collected during a preset period (suchas during a stress test) is used by the programmer to automaticallyselect detection parameters for the ST shift detection algorithm.

[0072] Another object of the present invention is to have a Guardiansystem with at least two levels of severity of patient alarm/alertingwhere the more severe EMERGENCY alarm alerts the patient to seekimmediate medical attention.

[0073] Another object of the present invention is to have a cardiacevent detected by comparing at least one heart signal parameterextracted from an electrogram segment captured at a first predeterminedtime by an implantable cardiosaver with the same at least one heartsignal parameter extracted from an electrogram segment captured at asecond predetermined time.

[0074] Another object of the present invention is to have acutemyocardial infarction detected by comparing recent electrogram data tobaseline electrogram data from the same time of day (i.e. approximately24 hours in the past).

[0075] Another object of the present invention is to have acutemyocardial infarction detected by comparing the ST deviation of thebeats in a recently collected electrogram segment to the average STdeviation of two or more beats of a baseline electrogram segment.Another object of the present invention is to have acute myocardialinfarction detected by comparing the ST segment voltage of the beats ina recently collected electrogram segment to the average ST segmentvoltage of two or more beats of a baseline electrogram segment.

[0076] Another object of the present invention is to have thethreshold(s) for detecting the occurrence of a cardiac event adjusted bya cardiosaver device to compensate for slow changes in the averagesignal level of the patient's electrogram.

[0077] Another object of the present invention is to have the thresholdfor detection of a cardiac event adjusted by a cardiosaver device tocompensate for daily cyclic changes in the average signal level of thepatient's electrogram.

[0078] Another object of the present invention is to have an externalalarm system including an alarm off button that will turn off either orboth internal and external alarm signals initiated by an implantedcardiosaver.

[0079] Another object of the present invention is to have the alarmsignal generated by a cardiosaver automatically turn off after a presetperiod of time.

[0080] Still another object of this invention is to use the cardiosaverto warn the patient that an acute myocardial infarction has occurred bymeans of a subcutaneous vibration.

[0081] Still another object of this invention is to have the cardiacevent detection require that at least a majority of the beats exhibit anexcessive ST shift before identifying an acute myocardial infarction.

[0082] Still another object of this invention is to have the cardiacevent detection require that excessive ST shift still be present in atleast two electrogram segments separated by a preset period of time.

[0083] Still another object of this invention is to have the cardiacevent detection require that excessive ST shift still be present in at.least three electrogram segments separated by preset periods of time.

[0084] Yet another object of the present invention is to have athreshold for detection of excessive ST shift that is dependent upon theaverage signal strength calculated from a baseline electrogram segment.

[0085] Yet another object of the present invention is to have athreshold for detection of excessive ST shift that is a function of thedifference between the average PQ segment amplitude and the R wave peakamplitude of a baseline electrogram segment.

[0086] Yet another object of the present invention is to have athreshold for detection of excessive ST shift that is a function of theaverage minimum to maximum (peak-to-peak) voltage for at least two beatscalculated from a baseline electrogram segment.

[0087] Yet another object of the present invention is to have theability to detect a cardiac event by the shift in the amplitude of the Twave of an electrogram segment at a second predetermined time ascompared with the average baseline T wave amplitude from a baselineelectrogram segment at a first predetermined time.

[0088] Yet another object of the present invention is to have theability to detect a cardiac event by the shift in the T wave deviationof at least one beat of an electrogram segment at a second predeterminedtime as compared with the average baseline T wave deviation from anelectrogram segment at a first predetermined time.

[0089] Yet another object of the present invention is to have the firstand second predetermined times for T wave amplitude and/or deviationcomparison be separated by less than 30 minutes.

[0090] Yet another object of the present invention is to have thebaseline electrogram segment used for ST segment shift detection and thebaseline electrogram segment used for T wave shift detection becollected at different times.

[0091] Yet another object of the present invention is to have an initialalarm-on patient alerting period followed by a reminder alarm thatperiodically cycles on and off over a periodic reminder alarm period.

[0092] Yet another object of the present invention is to have anindividualized (patient specific) “normal” heart rate range such thatthe upper and lower limits of “normal” are programmable using thecardiosaver programmer.

[0093] Yet another object of the present invention is to have one ormore individualized (patient specific) “elevated” heart rate ranges suchthat the upper and lower limits of each “elevated” range areprogrammable using the cardiosaver programmer.

[0094] Yet another object of the present invention is to allow thethreshold for detection of an excessive ST shift be different for the“normal” heart rate range as compared to one or more “elevated” heartrate ranges.

[0095] Yet another object of the present invention is to allow real timeor near real time display of electrogram data for diagnostic purposes.

[0096] Yet another object of the present invention is to have the timeperiod between collections of electrogram data vary, where the timeperiod is lengthened when the electrogram is normal and shortened whenthe electrogram is abnormal.

[0097] Yet another object of the present invention is to have differentcriteria for the normal/abnormal electrogram decision that influencesthe time period between collections of electrogram data as compared withthe criteria for detecting a cardiac event.

[0098] These and other objects and advantages of this invention willbecome obvious to a person of ordinary skill in this art upon reading ofthe detailed description of this invention including the associateddrawings as presented herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0099]FIG. 1 illustrates a guardian system for the detection of acardiac event and for warning the patient that a cardiac event isoccurring.

[0100]FIG. 2 illustrates a normal electrogram pattern and also shows asuperimposed elevated ST segment that would be indicative of an acutemyocardial infarction.

[0101]FIG. 3 is a plan view of the cardiosaver showing the cardiosaverelectronics module and two electrical leads each having one electrode.

[0102]FIG. 4 is a block diagram of the cardiosaver.

[0103]FIG. 5 is a block diagram of the cardiosaver event detectionprogram.

[0104]FIG. 6 illustrates the extracted electrogram segment features usedto calculate ST shift.

[0105]FIG. 7 is a block diagram of the baseline parameter extractionsubroutine of the cardiosaver event detection program.

[0106]FIG. 8 is a block diagram of the alarm subroutine of thecardiosaver event detection program.

[0107]FIG. 9 is a block diagram of the hi/low heart rate subroutine ofthe cardiosaver event detection program.

[0108]FIG. 10 is a block diagram of the ischemia subroutine of thecardiosaver event detection program

[0109]FIG. 11 is a diagram of the conditions that trigger cardiosaveralarms.

[0110]FIG. 12 is a block diagram of the unsteady heart rate subroutineof the cardiosaver event detection program.

[0111]FIG. 13 is an alternate embodiment of the guardian system.

[0112]FIG. 14 illustrates the preferred physical embodiment of theexternal alarm transceiver.

[0113]FIG. 15 illustrates the physical embodiment of the combinedexternal alarm transceiver and pocket PC.

DETAILED DESCRIPTION OF THE INVENTION

[0114]FIG. 1 illustrates one embodiment of the guardian system 10consisting of an implanted cardiosaver 5 and external equipment 7. Thebattery powered cardiosaver 5 contains electronic circuitry that candetect a cardiac event such as an acute myocardial infarction orarrhythmia and warn the patient when the event occurs. The cardiosaver 5can store the patient's electrogram for later readout and can sendwireless signals 53 to and receive wireless signals 54 from the externalequipment 7. The functioning of the cardiosaver 5 will be explained ingreater detail with the assistance of FIG. 4.

[0115] The cardiosaver 5 has two leads 12 and 15 that have multi-wireelectrical conductors with surrounding insulation. The lead 12 is shownwith two electrodes 13 and 14. The lead 15 has subcutaneous electrodes16 and 17. In fact, the cardiosaver 5 could utilize as few as one leador as many as three and each lead could have as few as one electrode oras many as eight electrodes. Furthermore, electrodes 8 and 9 could beplaced on the outer surface of the cardiosaver 5 without any wires beingplaced externally to the cardiosaver 5.

[0116] The lead 12 in FIG. 1 could advantageously be placed through thepatient's vascular system with the electrode 14 being placed into theapex of the right ventricle. The lead 12 with electrode 13 could beplaced in the right ventricle or right atrium or the superior vena cavasimilar to the placement of leads for pacemakers and ImplantableCoronary Defibrillators (ICDs). The metal case 11 of the cardiosaver 5could serve as an indifferent electrode with either or both electrodes13 and/or 14 being active electrodes. It is also conceived that theelectrodes 13 and 14 could be used as bipolar electrodes. Alternately,the lead 12 in FIG. 1 could advantageously be placed through thepatient's vascular system with the electrode 14 being placed into theapex of the left ventricle. The electrode 13 could be placed in the leftatrium.

[0117] The lead 15 could advantageously be placed subcutaneously at anylocation where the electrodes 16 and/or 17 would provide a goodelectrogram signal indicative of the electrical activity of the heart.Again for this lead 15, the case 11 of the cardiosaver 5 could be anindifferent electrode and the electrodes 16 and/or 17 could be activeelectrodes or electrodes 16 and 17 could function together as bipolarelectrodes. The cardiosaver 5 could operate with only one lead and asfew as one active electrode with the case of the cardiosaver 5 being anindifferent electrode. The guardian system 10 described herein canreadily operate with only two electrodes.

[0118] One embodiment of the cardiosaver device 5 using subcutaneouslead 15 would have the electrode 17 located under the skin on thepatient's left side. This could be best located between 2 and 20 inchesbelow the patient's left arm pit. The cardiosaver case 11 could act asthe indifferent electrode and would typically be implanted under theskin on the left side of the patient's chest.

[0119]FIG. 1 also shows the external equipment 7 that consists of aphysician's programmer 68 having an antenna 70, an external alarm system60 including a charger 166. The external equipment 7 provides means tointeract with the cardiosaver 5. These interactions include programmingthe cardiosaver 5, retrieving data collected by the cardiosaver 5 andhandling alarms generated by the cardiosaver 5.

[0120] The purpose of the physician's programmer 68 shown in FIG. 1 isto set and/or change the operating parameters of the implantablecardiosaver 5 and to read out data stored in the memory of thecardiosaver 5 such as stored electrogram segments. This would beaccomplished by transmission of a wireless signal 54 from the programmer68 to the cardiosaver 5 and receiving of telemetry by the wirelesssignal 53 from the cardiosaver 5 to the programmer 68. When a laptopcomputer is used as the physician's programmer 68, it would requireconnection to a wireless transceiver for communicating with thecardiosaver 5. Such a transceiver could be connected via a standardinterface such as a USB, serial or parallel port or it could be insertedinto the laptop's PCMCIA card slot. The screen on the laptop would beused to provide guidance to the physician in communicating with thecardiosaver 5. Also, the screen could be used to display both real timeand stored electrograms that are read out from the cardiosaver 5.

[0121] In FIG. 1, the external alarm system 60 has a patient operatedinitiator 55, an alarm disable button 59, a panic button 52, an alarmtransceiver 56, an alarm speaker 57 and an antenna 161 and cancommunicate with emergency medical services 67 with the modem 165 viathe communication link 65.

[0122] If a cardiac event is detected by the cardiosaver 5, an alarmmessage is sent by a wireless signal 53 to the alarm transceiver 56 viathe antenna 161. When the alarm is received by the alarm transceiver 56a signal 58 is sent to the loudspeaker 57. The signal 58 will cause theloudspeaker to emit an external alarm signal 51 to warn the patient thatan event has occurred. Examples of external alarm signals 51 include aperiodic buzzing, a sequence of tones and/or a speech message thatinstructs the patient as to what actions should be taken. Furthermore,the alarm transceiver 56 can, depending upon the nature of the signal53, send an outgoing signal over the link 65 to contact emergencymedical services 67. When the detection of an acute myocardialinfarction is the cause of the alarm, the alarm transceiver 56 couldautomatically notify emergency medical services 67 that a heart attackhas occurred and an ambulance could be sent to treat the patient and tobring him to a hospital emergency room.

[0123] If the remote communication with emergency medical services 67 isenabled and a cardiac event alarm is sent within the signal 53, themodem 165 will establish the data communications link 65 over which amessage will be transmitted to the emergency medical services 67. Themessage sent over the link 65 may include any or all of the followinginformation: (1) a specific patient is having an acute myocardialinfarction or other cardiac event, (2) the patient's name, address and abrief medical history, (3) a map and/or directions to where the patientis located, (4) the patient's stored electrogram including baselineelectrogram data and the specific electrogram segment that generated thealarm (5) continuous real time electrogram data, and (6) a prescriptionwritten by the patient's personal physician as to the type and amount ofdrug to be administered to the patient in the event of a heart attack.If the emergency medical services 67 includes an emergency room at ahospital, information can be transmitted that the patient has had acardiac event and should be on his way to the emergency room. In thismanner the medical practitioners at the emergency room could be preparedfor the patient's arrival.

[0124] The communications link 65 can be either a wired or wirelesstelephone connection that allows the alarm transceiver 56 to call out toemergency medical services 67. The typical external alarm system 60might be built into a Pocket PC or Palm Pilot PDA where the alarmtransceiver 56 and modem 165 are built into insertable cards having astandardized interface such as compact flash cards, PCMCIA cards,multimedia, memory stick or secure digital (SD) cards. The modem 165 canbe a wireless modem such as the Sierra AirCard 300 or the modem 165 maybe a wired modem that connects to a standard telephone line. The modem165 can also be integrated into the alarm transceiver 56.

[0125] The purpose of the patient operated initiator 55 is to give thepatient the capability for initiating transmission of the most recentlycaptured electrogram segment from the cardiosaver 5 to the externalalarm system 60. This will enable the electrogram segment to bedisplayed for a medical practitioner.

[0126] Once an internal and/or external alarm signal has been initiated,depressing the alarm disable button 59 will acknowledge the patient'sawareness of the alarm and turn off the internal alarm signal generatedwithin the cardiosaver 5 and/or the external alarm signal 51 playedthrough the speaker 57. If the alarm disable button 59 is not used bythe patient to indicate acknowledgement of awareness of a SEE DOCTORalert or an EMERGENCY alarm, it is envisioned that the internal and/orexternal alarm signals would stop after a first time period (an initialalarm-on period) that would be programmable through the programmer 68.

[0127] For EMERGENCY alarms, to help prevent a patient ignoring orsleeping through the alarm signals generated during the initial alarm-onperiod, a reminder alarm signal might be turned on periodically during afollow-on periodic reminder time period. This periodic reminder time istypically much longer than the initial alarm-on period. The periodicreminder time period would typically be 3 to 5 hours because after 3 to5 hours the patient's advantage in being alerted to seek medicalattention for a severe cardiac event like an AMI is mostly lost. It isalso envisioned that the periodic reminder time period could also beprogrammable through the programmer 68 to be as short as 5 minutes oreven continue indefinitely until the patient acknowledges the alarmsignal with the button 59 or the programmer 68 is used to interact withthe cardiosaver 5.

[0128] Following the initial alarm on-period there would be an alarmoff-period followed by a reminder alarm on-period followed by an alarmoff-period followed by another reminder alarm on-period and so onperiodically repeating until the end of the periodic reminder timeperiod.

[0129] The alarm off-period time interval between the periodic remindersmight also increase over the reminder alarm on-period. For example, theinitial alarm-on period might be 5 minutes and for the first hourfollowing the initial alarm-on period, a reminder signal might beactivated for 30 seconds every 5 minutes. For the second hour thereminder alarm signal might be activated for 20 seconds every 10 minutesand for the remaining hours of the periodic reminder on-period thereminder alarm signal might be activated for 30 seconds every 15minutes.

[0130] The patient might press the panic button 52 in the event that thepatient feels that he is experiencing a cardiac event. The panic button52 will initiate the transmission from the cardiosaver 5 to the externalalarm system 60 via the wireless signal 53 of both recent and baselineelectrogram segments. The external alarm system 60 will then retransmitthese data via the link 65 to emergency medical services 67 where amedical practitioner will view the electrogram data. The remote medicalpractitioner could then analyze the electrogram data and call thepatient back to offer advice as to whether this is an emergencysituation or the situation could be routinely handled by the patient'spersonal physician at some later time.

[0131] It is envisioned that there may be preset limits within theexternal alarm system 60 that prevent the patient operated initiator 55and/or panic button from being used more than a certain number of timesa day to prevent the patient from running down the batteries in thecardiosaver 5 and external alarm system 60 as wireless transmissiontakes a relatively large amount of power as compared with otherfunctional operation of these devices.

[0132]FIG. 2 illustrates a typical electrogram signal having beats 1 and2 from some pair of implanted electrodes such as the electrode 14 andthe case 11 of FIG. 3 overlaid with an electrogram having an elevated STsegment 4 (dashed line). The various portions of the electrogram areshown as the P, Q, R, S, and T waves. These are all shown as portions ofa solid line in FIG. 2. The normal ST segment 3 of beat 2 is also shownin FIG. 2. The R-R interval 5 for beat 2 is shown as the time betweenthe R waves of beat 2 and the beat before it (beat 1).

[0133] When an acute myocardial infarction occurs, there is typically anelevation (or depression) of the ST segment 4 as shown by the dashedline in FIG. 2. It is this shift of the ST segment 4 as compared to thebaseline ST segment 3 that is a clear indicator that an acute myocardialinfarction has occurred in a significant portion of the patient'smyocardium.

[0134] Although an elevated ST segment 4 can be a good indicator of anacute myocardial infarction, other indicators such as a sudden change ofheart rate or heart wall motion, intra-coronary blood pressure or asudden decrease in blood P0 ₂ could also be used as independent sensingmeans or those signals could be used in addition to the voltage shift ofthe ST segment 4.

[0135] It is important to note that the electrogram from implantedelectrodes may provide a faster detection of an ST segment shift ascompared to an electrocardiogram signal obtained from skin surfaceelectrodes. Thus the electrogram from implanted electrodes as describedherein is the preferred embodiment of the present invention.

[0136] It is also well known that the T wave can shift very quickly whena heart attack occurs. It is envisioned that the present invention mightdetect this T wave shift as compared to a time of 1 to 5 minutes in thepast.

[0137] It is anticipated that when a patient who has a stenosis in acoronary artery is performing a comparatively strenuous exercise hisheart rate increases and he can develop exercise induced ischemia thatwill also result in a shift of the ST segment of his electrogram. Thisis particularly true for patients who have undergone balloon angioplastywith or without stent implantation. Such patients will be informed bytheir own physician that, if their cardiosaver 5 of FIG. 1 activates analarm during exercise, that it may be indicative of the progression ofan arterial stenosis in one of the heart's arteries. Such a patientwould be advised to stop all exertion immediately and if the alarmsignal goes away as his heart rate slows, the patient should see hisdoctor as soon as convenient. If the alarm signal does not go away asthe patient's heart rate slows down into the normal range then thecardiosaver will change the alarm signal to indicate that the patientshould immediately seek medical care. As previously described, thecardiosaver 5 could emit a different signal if there is a heart attackas compared to the signal that would be produced if there were ischemiaresulting from exercise.

[0138] It is also envisioned that heart rate and the rate of change ofheart rate experienced during an ST segment voltage shift can be used toindicate which alarm should be produced by the cardiosaver 5.Specifically, an ST segment shift at a near normal heart rate wouldindicate an acute myocardial infarction. An ST segment shift when thereis an elevated heart rate (e.g., greater than 100 bpm) would generallybe indicative of a progressing stenosis in a coronary artery. In anycase, if a sufficient ST segment shift occurs that results in an alarmfrom the cardiosaver 5, the patient should promptly seek medical care todetermine the cause of the alarm.

[0139] It should be understood that, depending on a patient's medicalcondition, a vigorous exercise might be as energetic as running a longdistance or merely going up a flight of stairs. After the cardiosaver 5is implanted in a patient who has undergone a stent implant, he shouldhave a stress test to determine his level of ST segment shift that isassociated with the highest level of exercise that he can attain. Thepatient's heart rate should then be noted and the cardiosaver thresholdsfor detection, described with FIGS. 5 through 9, should be programmed soas to not alarm at ST segment shifts observed during exercise. Then ifat a later time the patient experiences an increased shift of his STsegment at that pre-determined heart rate or within a heart rate range,then an alarm indicating ischemia can be programmed to occur. Theoccurrence of such an alarm can indicate that there is a progression inthe narrowing of some coronary artery that may require angiography todetermine if angioplasty, possibly including stent implantation, isrequired.

[0140] The alarm signal associated with an excessive ST shift caused byan acute myocardial infarction can be quite different from the “SEEDOCTOR” alarm means associated with progressing ischemia duringexercise. For example, the SEE DOCTOR alert signal might be an audiosignal that occurs once every 5 to 10 seconds. A different alarm signal,for example an audio signal that is three buzzes every 3 to 5 seconds,may be used to indicate a major cardiac event such as an acutemyocardial infarction. Similar alarm signal timing would typically beused for both internal alarm signals generated by the alarm sub-system48 of FIG. 4 and external alarm signals generated by the external alarmsystem 60.

[0141] In any case, a patient can be taught to recognize which signaloccurs for these different circumstances so that he can take immediateresponse if an acute myocardial infarction is indicated but can take anon-emergency response if progression of the narrowing of a stenosis orsome other less critical condition is indicated. It should be understoodthat other distinctly different audio alarm patterns could be used fordifferent arrhythmias such as atrial fibrillation, atrial flutter,PVC's, PAC's, etc. A capability of the physician's programmer 68 of FIG.1 would be to program different alarm signal patterns, enable or disabledetection and/or generation of associated alarm signals in thecardiosaver for any one or more of these various cardiac events. Also,the intensity of the audio alarm, vibration or electrical tickle alarmcould be adjusted to suit the needs of different patients. In order tofamiliarize the patient with the different alarm signals, the programmer68 of the present invention would have the capability to turn each ofthe different alarm signals on and off.

[0142]FIG. 3 is a plan view of the cardiosaver 5 having a case 11 and aplastic header 20. The case 11 contains the primary battery 22 and theelectronics module 18. This type of package is well known forpacemakers, implantable defibrillators and implantable tissuestimulators. Electrical conductors placed through the plastic header 20connect the electronics module 18 to the electrical leads 12 and 15,which have respectively electrodes 14 and 17. The on-case electrodes 8and 9 of FIG. 1 are not shown in FIG. 3. It should also be understoodthat the cardiosaver 5 can function with only two electrodes, one ofwhich could be the case 11. All the different configurations forelectrodes shown in FIGS. 1 and 3, such as the electrodes 8, 9, 13, 14,16 or the metal case 11 are shown only to indicate that there are avariety of possible electrode arrangements that can be used with thecardiosaver 5.

[0143] On the metal case 11, a conducting disc 31 mounted onto aninsulating disc 32 can be used to provide a subcutaneous electricaltickle to warn the patient that an acute myocardial infarction isoccurring or to act as an independent electrode.

[0144]FIG. 4 is a block diagram of the cardiosaver 5 with primarybattery 22 and a secondary battery 24. The secondary battery 24 istypically a rechargeable battery of smaller capacity but higher currentor voltage output than the primary battery 22 and is used for short termhigh output components of the cardiosaver 5 like the RF chipset in thetelemetry sub-system 46 or the vibrator 25 attached to the alarmsub-system 48. An important feature of the present invention.cardiosaver is the dual battery configuration where the primary battery22 will charge the secondary battery 24 through the charging circuit 23.The primary battery 22 is typically a larger capacity battery than thesecondary battery 24. The primary battery also typically has a lowerself discharge rate as a percentage of its capacity than the secondarybattery 24. It is also envisioned that the secondary battery could becharged from an external induction coil by the patient or by the doctorduring a periodic check-up.

[0145] The electrodes 14 and 17 connect with wires 12 and 15respectively to the amplifier 36 that is also connected to the case 11acting as an indifferent electrode. As two or more electrodes 12 and 15are shown here, the amplifier 36 would be a multi-channel amplifier. Theamplified electrogram signals 37 from the amplifier 36 are thenconverted to digital signals 38 by the analog-to-digital converter 41.The digital electrogram signals 38 are buffered in theFirst-In-First-Out (FIFO) memory 42. Processor means shown in FIG. 4 asthe central processing unit (CPU) 44 coupled to memory means shown inFIG. 4 as the Random Access Memory (RAM) 47 can process the digitalelectrogram data 38 stored the FIFO 42 according to the programminginstructions stored in the program memory 45. This programming (i.e.software) enables the cardiosaver 5 to detect the occurrence of acardiac event such as an acute myocardial infarction.

[0146] A clock/timing sub-system 49 provides the means for timingspecific activities of the cardiosaver 5 including the absolute orrelative time stamping of detected cardiac events. The clock/timingsub-system 49 can also facilitate power savings by causing components ofthe cardiosaver 5 to go into a low power standby mode in between timesfor electrogram signal collection and processing. Such cycled powersavings techniques are often used in implantable pacemakers anddefibrillators. In an alternate embodiment, the clock/timing sub-systemcan be provided by a program subroutine run by the central processingunit 44.

[0147] In an advanced embodiment of the present invention, theclock/timing circuitry 49 would count for a first period (e.g. 20seconds) then it would enable the analog-to-digital converter 41 andFIFO 42 to begin storing data, after a second period (e.g. 10 seconds)the timing circuitry 49 would wake up the CPU 44 from its low powerstandby mode. The CPU 44 would then process the 10 seconds of data in avery short time (typically less than a second) and go back to low powermode. This would allow an on off duty cycle of the CPU 44 which oftendraws the most power of less than 2 seconds per minute while actuallycollecting electrogram data for 20 seconds per minute.

[0148] In a preferred embodiment of the present invention the RAM 47includes specific memory locations for 3 sets of electrogram segmentstorage. These are the recent electrogram storage 472 that would storethe last 2 to 10 minutes of recently recorded electrogram segments sothat the electrogram data leading in the period just before the onset ofa cardiac event can be reviewed at a later time by the patient'sphysician using the physician's programmer 68 of FIG. 1. For example,the recent electrogram storage 472 might contain eight 10 second longelectrogram segments that were captured every 30 seconds over the last 4minutes.

[0149] The baseline electrogram memory 474 would provide storage forbaseline electrogram segments collected at preset times over one or moredays. For example, the baseline electrogram memory 474 might contain 24baseline electrogram segments of 10 seconds duration, one from each hourfor the last day.

[0150] The event memory 476 occupies the largest part of the RAM 47. Theevent memory 476 is not overwritten on a regular schedule as are therecent electrogram memory 472 and baseline electrogram memory 474 but istypically maintained until read out by the patient's physician with theprogrammer 68 of FIG. 1. At the time a cardiac event like excessive STshift indicating an acute myocardial infarction is detected by the CPU44, all (or part) of the entire contents of the baseline and recentelectrogram memories 472 and 474 would typically be copied into theevent memory 476 so as to save the pre-event data for later physicianreview.

[0151] In the absence of events, the event memory 476 could be usedtemporarily to extend the recent electrogram memory 472 so that moredata (e.g. every 10 minutes for the last 12 hours) could be held by thecardiosaver 5 of FIG. 1 to be examined by a medical practitioner at thetime a patient visits. This would typically be overwritten with pre- andpost-event electrogram segments following a detected event.

[0152] An example of use of the event memory 476 would have a SEE DOCTORalert saving the last segment that triggered the alarm and the baselineused by the detection algorithm in detecting the abnormality. AnEMERGENCY ALARM would save the sequential segments that triggered thealarm, a selection of other pre-event electrogram segments, or aselection of the 24 baseline electrogram segments and post-eventelectrogram segments. For example, the pre-event memory would havebaselines from −24 hrs, −18, −12, −6, −5, −4, −3, −2 and −1 hours,recent electrogram segments (other than the triggering segments) from −5minutes, −10, −20, −35, and −50 minutes, and post-event electrogramsegments for every 5 minutes for the 2 hours following the event and forevery 15 minutes after 2 hours post-event. These settings could bepre-set or programmable. The RAM 47 also contains memory sections forprogrammable parameters 471 and calculated baseline data 475. Theprogrammable parameters 471 include the upper and lower limits for thenormal and elevated heart rate ranges, and physician programmedparameters related to the cardiac event detection processes stored inthe program memory 45. The calculated baseline data 475 containdetection parameters extracted from the baseline electrogram segmentsstored in the baseline electrogram memory 474. Calculated baseline data475 and programmable parameters 471 would typically be saved to theevent memory 476 following the detection of a cardiac event. The RAM 47also includes patient data 473 that may include the patient's name,address, telephone number, medical history, insurance information,doctor's name, and specific prescriptions for different medications tobe administered by medical practitioners in the event of differentcardiac events.

[0153] It is envisioned that the cardiosaver 5 could also containpacemaker circuitry 170 and/or defibrillator circuitry 180 similar tothe cardiosaver systems described by Fischell in U.S. pat. No.6,240,049.

[0154] The alarm sub-system 48 contains the circuitry and transducers toproduce the internal alarm signals for the cardiosaver 5. The internalalarm signal can be a mechanical vibration, a sound or a subcutaneouselectrical tickle or shock.

[0155] The telemetry sub-system 46 with antenna 35 provides thecardiosaver 5 the means for two-way wireless communication to and fromthe external equipment 7 of FIG. 1. Existing radiofrequency transceiverchip sets such as the Ash transceiver hybrids produced by RFMicrodevices, Inc. can readily provide such two-way wirelesscommunication over a range of up to 10 meters from the patient. It isalso envisioned that short range telemetry such as that typically usedin pacemakers and defibrillators could also be applied to thecardiosaver 5. It is also envisioned that standard wireless protocolssuch as Bluetooth and 802.11a or 802.11b might be used to allowcommunication with a wider group of peripheral devices.

[0156] A magnet sensor 190 may be incorporated into the cardiosaver 5.An important use of the magnet sensor 190 is to turn on the cardiosaver5 on just before programming and implantation. This would reduce wastedbattery life in the period between the times that the cardiosaver 5 ispackaged at the factory until the day it is implanted.

[0157] The cardiosaver 5 might also include an accelerometer 175. Theaccelerometer 174 together with the processor 44 is designed to monitorthe level of patient activity and identify when the patient is active.The activity measurements are sent to the processor 44. In thisembodiment the processor 44 can compare the data from the accelerometer175 to a preset threshold to discriminate between elevated heart rateresulting from patient activity as compared to other causes.

[0158]FIG. 5 illustrates in the form of a block diagram the operation ofthe heart signal processing program 450 for cardiac event detection bythe cardiosaver 5 of FIGS. 1-4. The heart signal processing program 450is an example of one of many such detection programs whose instructionscould reside in the program memory 45 for use by the CPU 44 of thecardiosaver 5 as shown in FIG. 4. The main section of the heart signalprocessing program 450 begins with step 451 where the event counter “k”is set to zero indicating there have been no detected events. Next, instep 452 the cardiosaver 5 is said to sleep for X seconds. The termsleep here indicates that for a period of X seconds, the cardiosaver 5would either be placed in a low power standby mode (if available) orwould otherwise simply wait for a time of X seconds before moving tostep 453. Step 453 following 452 has an electrogram segment representingY seconds of electrogram data captured into the FIFO buffer 42 of FIG.4. σ a is the data sampling rate in samples per second, thus the totalnumber of samples collected in step 453 is σ multiplied by Y. It isenvisioned that X would be a time between 5 seconds and 5 minutes with20 seconds as a preferred value. Y would be between 3 and 30 secondswith 10 seconds as a preferred value. σ is typically between 100 and 500samples per second with 200 samples per second being a preferred value.

[0159] After being captured, in step 454, the Y seconds of electrogramdata representing the most recent electrogram segment is transferred tothe recent electrogram memory 472 of FIG. 4. At this time the processingand analysis of the data begins. Throughout the remainder of thisdetailed description of the drawings, the “Y second long electrogramsegment” refers to the most recently collected Y seconds of electrogramdata that have been captured and transferred to the recent electrogrammemory 472 by the steps 453 and 454. The term “recent electrogramsegments” refers to all of the electrogram segments stored in the recentelectrogram memory 472. For example, there could be eight total 10second long recent electrogram segments that were captured at 30 secondintervals over a 4 minute period.

[0160] The first processing step following the collection of the Ysecond long electrogram segment is step 455 that measures the intervalsbetween the R waves in the most Y second long electrogram segment. TheseR-R intervals are then used to calculate the average heart rate and R-Rinterval variation for the Y second long electrogram segment. If theaverage heart rate is below a programmed low heart rate limit ρ_(low) orabove a programmed high heart rate limit ρ_(high), it is considered“out-of-range” and a Hi/Low heart rate subroutine 420 (see FIG. 9) isrun to properly respond to the condition.

[0161] If the R-R interval variation within the Y second longelectrogram segment is more than a programmed limit, the hi/low heartrate subroutine is also run. This is an important feature of the presentinvention as PVC's and unstable heart rhythms such as a bigeminal rhythmcan cause errors in an ST shift detection algorithm that is works bestwith a steady heart rhythm. One embodiment of the present inventionidentifies an unsteady heart rate by comparing the two shortest R-Rintervals and the 2 longest intervals in the Y second long electrogramsegment. If the difference between both of the two shortest R-Rintervals and the average of the two longest R-R intervals are more thana programmed percentage α, an unsteady heart rate is identified. Forexample the programmed percentage α might be 25% so that if the twoshortest R-R intervals are each more than 25% less than the average ofthe two longest R-R intervals, then the heart rate is unsteady. It isenvisioned that if longer times Y are used for electrogram segmentcollection then it might require 3 or more “short ” beats to indicatedan unsteady heart rate. Any beat that is not too short is classified bystep 455 as a normal beat. ρ_(low), ρ_(high) and a are programmableparameters typically set using the programmer 68 during programming ofthe cardiosaver 5. Typical values for ρ_(low) and ρ_(high) would be 50and 140 beats per minute respectively.

[0162] If the heart rate is not high, low or unsteady as checked in step455, the heart signal processing program 450 moves to step 456 where theaverage heart rate is compared to a programmed normal range betweenρ_(low) and ρ_(elevated) where ρ_(elevated) is the elevated heart ratelimit that defines the upper limit of the “normal range” (e.g. 80 beatsper minute). If the patient's heart rate is elevated but notout-of-range (i.e. above ρ_(high)), the patient may be exercising andthe ischemia subroutine 480 allows for different cardiac event detectioncriteria during elevated heart rates to reduce false positive detectionsof acute myocardial infarction and to detect exercise induced ischemia.An example of one embodiment of the ischemia subroutine 480 isillustrated in FIG. 10.

[0163] Although the above specification describes low, high and elevatedheart rate limits ρ_(low), ρ_(high) and ρ_(elevated), it is envisionedthat instead of heart rate (i.e. beats per second) the limits anddecision making could be set in terms or R wave to R wave (R-R) intervalwith the low, high and elevated limits are for R-R interval and areexpressed in seconds per beat, milliseconds per beat or samples perbeat.

[0164] If the average heart rate of the patient is within the “normal”range in step 456, then the program 450 moves to step 457 where it looksfor an excessive ST shift on M out of N beats as compared with thebaseline electrogram segment collected at a time U±W minutes in thepast. U can be any time from 1 minute to 48 hours but to allow for dailycycles U=24 hours is a preferred embodiment. W is half the intervalbetween times when the baseline data is saved and can be any time from10 seconds to 12 hours. For a U of 24 hours, a preferred setting wouldhave W equal to half an hour so that the current Y second longelectrogram segment is always being compared with a baseline electrogramsegment from {fraction (24±1/2)} hour before. This also means thatbaseline electrogram segments are saved and processed to extractdetection parameters at an interval of twice W (2W). I.e., if W is halfan hour, then the baseline data is saved and processed once an hour. Mcan be any number from 1 to 30 and N can be any number from M to 100. Anexample of a typical M and N used would be 6 out of 8 beats. It isenvisioned that the first of the 8 beats will typically be the beatincluding the 2^(nd) R wave in the Y second long electrogram segmentcollected in steps 453 and 454.

[0165] If one is trying to detect abnormalities in 6 out of 8 beats fora positive detection, a negative detection will occur whenever 3 OKbeats without a detected abnormality are found (so long as it is beforethe 6 “abnormal” beats with detected abnormalities). To save processingtime and potentially extend battery life it is desirable to have steps457 and 469 of FIG. 5 simultaneously count both the number of OK beatsand the number of abnormal beats. The steps 457 and 469 will stopprocessing beats when either 3 OK beats (a negative detection) or 6abnormal beats (a positive detection) are found. Another advantage ofthis technique is that even if the Y second long electrogram segmentscollected in steps 453 and 465 have less than 6 beats but there are atleast 3 OK beats, there sufficient data to declare a negative detection(i.e. nothing is wrong). As heart attacks occur rarely, this improvementwill greatly enhance the efficiency of detection algorithm. Although theexample above uses 3 OK vs. 6 out of 8 abnormal beats, this techniquewill work for any M out of N detection scheme where N−M+1 OK beats issufficient to declare that no event has occurred. This enhancement willwork in any device for detecting cardiac events whether implanted withinthe patient or external to the patient. This technique both looking forOK and abnormal beats can be applied throughout the subroutines of thepresent invention. For example, ST shift is detected in steps 434 and439 of FIG. 9 and is of particular importance with a low heart ratewhere there may not be M beats to process in the Y seconds. It is alsoapplicable to the Unsteady Heart Rate Subroutine 410 in step 418 and canreduce the number of times that an additional Y second electrogramsegment must be collected to get sufficient data to detect the presenceor absence of an event.

[0166] The electrogram segment length Y should be programmed to be ofsufficient length such that there will be more than N beats within the Ysecond electrogram segment for heart rates at the low limit for thenormal heart rate range. If Y is too short, then the programs 450 and460 may need to also allow for the collection of additional electrogramdata as shown in FIG. 12 for the unsteady heart rate subroutine 410.

[0167] An alternate to ST shift detection in step 457 is to process justthe T wave, which can change its peak or average amplitude rapidly ifthere is a heart attack. The T wave can, however change its amplitudeslowly under normal conditions so a T wave shift detector would need amuch shorter time U than that of a detector using the ST segment beforethe T wave. If the detector is checking for such T wave shift, i.e. avoltage shift of the T wave part of the ST segment, then it may bedesirable to check against a baseline where U is 1 to 30 minutes and Wis 15 seconds to 15 minutes. For example, U=3 minutes and W=15 secondsis a preferred setting to catch a quickly changing T wave. This wouldalso allow use of recent electrogram segments stored in the recentelectrogram memory of FIG. 4 as baseline electrogram segments for T waveshift detection. It is envisioned that the programmer 68 of FIG. 1 wouldallow the patient's doctor to program the cardiosaver 5 to use STsegment shift or T wave shift detectors by themselves, or togethersimultaneously. If both were used then the programmer 68 would allow thepatient's doctor to choose whether a positive detection will result ifeither technique detects an event or only if both detect an event.

[0168] If the average heart rate is in the normal range, is not unsteadyand there is no cardiac event detection in step 457, (i.e. theelectrogram signal is indicative of a “normal” heart signal for thepatient), the heart signal processing program 450 checks in step 458 ifit is more than the interval of 2W minutes since the last time baselinedata was captured. If it has been more than 2W, the baseline parameterextraction subroutine 440 of FIG. 7 is run.

[0169] The parameters X, Y, U and W are stored with the programmableparameters 471 in the RAM 47 in FIG. 4. These parameters may bepermanently set at the time of manufacturing of the cardiosaver 5 orthey may be programmed through the programmer 68 of FIG. 1. Thecalculated criteria for cardiac event detection extracted from thebaseline electrogram segments stored in baseline electrogram memory 474are stored in the calculated baseline data memory 475 of the RAM 47.

[0170] A typical configuration of the heart signal processing program450 using only an ST shift detector, would use a sleep of X=20 seconds,followed by collection of a Y=10 second long electrogram segment. If thepatient's heart rate is in a normal range of between 50 and 80 beats perminute, step 457 would check for an excessive shift of the ST segment in6 out of 8 of the beats as compared with baseline data collected{fraction (24±1/2)} hour previously.

[0171] If there has been a detected excessive ST shift in M out of Nbeats in step 457, the ST Verification Subroutine 460 is run to be surethat the detected event is not a transitory change in the electrogram.

[0172] The ST Verification Subroutine 460 begins with step 461 where therecently collected Y second long electrogram segment is saved to theevent memory 476 of FIG. 4 for later review by the patient's doctor.

[0173] The ST shift verification subroutine 460 then increments theevent counter k by 1 (step 462) and then checks (step 463) if k is equalto 3 (i.e. 3 events is the trigger for an alarm. If k=3 then the alarmsubroutine 490 illustrated in FIG. 8 is run, thus declaring that therehas been a positive detection of a major cardiac event. FIG. 11illustrates examples of the combinations of conditions that can lead tok=3 and the running of the alarm subroutine 490.

[0174] Although step 463 is shown checking if k=3 as the condition forrunning the alarm subroutine 490, the number of events required could bea programmable parameter from k=1 to k=20. Even higher possible valuesthan k=20 might be used to avoid false positive detections. With currentaverage times from onset of a heart attack to arrival at a treatmentcenter of 3 hours, a few minutes delay for a device that should enablethe patient to easily reach a treatment center within 30 minutes isvaluable if it improves the reliability of detection.

[0175] In step 463 if k is less than 3 then the ST shift verificationsubroutine 460 proceeds to sleep Z seconds in step 464 followed bycollection (step 465) and saving (step 466) to the next location in therecent electrogram memory 472 of FIG. 4 of a new Y second longelectrogram segment. Z seconds can be different from the X seconds usedin step 452 to allow the ST shift verification subroutine 460 to lookover longer (or shorter) intervals than the main program so as to bestverify the positive detection of step 457. The term sleep here has thesame connotation as in step 452. A preferred embodiment of the presentinvention uses Z=X=20 seconds.

[0176] The ST shift verification subroutine 460 then checks for heartrate out-of-range or unsteady in step 467. As described with respect tostep 455 above, heart rate out-of-range means that the average heartrate in the Y second long electrogram segment is below the low heartrate limit ρ_(low) or above the high heart rate limit ρ_(high).

[0177] If the heart rate is out-of range or unsteady step 467 willinitiate the Hi/Low subroutine 420. If the heart rate is not out-ofrange or unsteady, then step 468 follows to check if the heart rate isnormal or elevated similar to step 456 above. If the heart rate iselevated, the ischemia subroutine 480 is run. The reason for checking ifthe heart rate has changed is that acute myocardial infarction caninduce high heart rates from tachycardia or fibrillation that might maskthe ST shift but are in of themselves major cardiac events whosedetection will increment the event counter k.

[0178] If the heart rate is in the normal range (i.e. not elevated),then step 469 checks for an excessive ST and/or T wave shift in M out ofN beats of the Y second long electrogram segment as compared with thebaseline data extracted U ±W minutes in the past (similar to step 457).If no excessive ST and/or T wave shift is seen, the subroutine 460returns to step 458 of the heart signal processing program 450 and theneventually back to step 451, the start of heart signal processingprogram 450. In step 451, k is set back to 0 so that only if there arecardiac events detected in three (k) successive Y second longelectrogram segments, will the alarm subroutine 490 be run. In apreferred embodiment of the present invention, steps 457 and 469 onlyexamine M out of N “normal” beats, ignoring any beats that are too shortas determined by step 455.

[0179] It is important to note, that baseline data is extracted onlywhen the heart rate is within the normal range and there is not anexcessive ST or T wave shift in M out of N beats. In one embodiment ofthe present invention, this is improved further by having the baselineparameter extraction subroutine 440 only process normal beats thatindividually do not exhibit an excessive ST and/or T wave shift.

[0180]FIG. 6 illustrates the features of a single normal beat 500 of anelectrogram segment and a single beat 500′ of an AMI electrogram segmentthat has a significant ST segment shift as compared with the normal beat500. Such ST segment shifting occurs within minutes following theocclusion of a coronary artery during an AMI. The beats 500 and 500′show typical heart beat wave elements labeled P, Q, R, S, and T. Thedefinition of a beat such as the beat 500 is a sub-segment of anelectrogram segment containing exactly one R wave and including the Pand Q elements before the R wave and the S and T elements following theR wave.

[0181] For the purposes of detection algorithms, different sub-segments,elements and calculated values related to the beats 500 and 500′ arehereby specified. The peak of the R wave of the beat 500 occurs at thetime T_(R) (509). The PQ segment 501 and ST segment 505 are sub-segmentsof the normal beat 500 and are located in time with respect to the timeT_(R) (509) as follows:

[0182] a. The PQ segment 501 has a time duration D_(PQ) (506) and startsT_(PQ) (502) milliseconds before the time T_(R) (509).

[0183] b. The ST segment 505 has a time duration D_(ST) (508) and startsT_(ST) (502) milliseconds after the time T_(R) (509).

[0184] The PQ segment 501′ and ST segment 505′ are sub-segments of thebeat 500′ and are located in time with respect to the time T′_(R)(509′)as follows:

[0185] c. The PQ segment 501′ has a time duration D_(PQ) (506) andstarts T_(PQ) (502) milliseconds before the time T′_(R) (509′).

[0186] d. The ST segment 505′ has a time duration D_(ST) (508) andstarts T_(ST) (502) milliseconds after the time T′_(R) (509′).

[0187] The ST segments 505 and 505′ and the PQ segments 501 and 501′ areexamples of sub-segments of the electrical signals from a patient'sheart. The R wave and T wave are also sub-segments. The dashed linesV_(PQ) (512) and V_(ST) (514) illustrate the average voltage amplitudesof the PQ and ST segments 501 and 505 respectively for the normal beat500. Similarly the dashed lines V′_(PQ) (512′) and V′_(ST) (514′)illustrate the average amplitudes of the PQ and ST segments 501′ and505′ respectively for the beat 500′. The “ST deviation” ΔV (510) of thenormal beat 500 and the ST deviation ΔV_(AMI) (510′) of the AMIelectrogram beat 500′ are defined as:

ΔV(510)=V _(ST)(514)−V _(PQ)(512)

ΔV _(AMI)(510′)=V′ _(ST)(514′)−V′ _(PQ) (512′)

[0188] Note that the both beats 500 and 500′ are analyzed using the sametime offsets T_(PQ) and T_(ST) from the peak of the R wave and the samedurations D_(PQ) and D_(ST). In this example, the beats 500 and 500′ areof the same time duration (i.e. the same heart rate). The parametersT_(PQ), T_(ST), D_(PQ) and D_(ST) would typically be set with theprogrammer 68 of FIG. 1 by the patient's doctor at the time thecardiosaver 5 is implanted so as to best match the morphology of thepatient's electrogram signal and normal heart rate. V_(PQ) (512), V_(ST)(514), V_(R) (503) and ΔV (510) are examples of per-beat heart signalparameters for the beat 500.

[0189] Although it may be effective to fix the values of time offsetsT_(PQ) (502) and T_(ST) (504) and the durations D_(PQ) (506) and D_(ST)(508), it is envisioned that the time offsets T_(PQ) and T_(ST) and thedurations D_(PQ) and D_(ST) could be automatically adjusted by thecardiosaver 5 to account for changes in the patient's heart rate. If theheart rate increases or decreases, as compared with the patient's normalheart rate, it envisioned that the offsets T_(PQ) (502) and T_(ST) (504)and/or the durations D_(PQ) (506) and D_(ST) (508) could vary dependingupon the R-R interval between beats or the average R-R interval for anelectrogram segment. A simple technique for doing this would vary theoffsets T_(PQ) and T_(ST) and the durations D_(PQ) and D_(ST) inproportion to the change in R-R interval. For example if the patient'snormal heart rate is 60 beats per minute, the R-R interval is 1 second;at 80 beats per minute the R-R interval is 0.75 seconds, a 25% decrease.This could automatically produce a 25% decrease in the values of T_(PQ),T_(ST), D_(PQ) and D_(ST). Alternately, the values for T_(PQ), T_(ST),D_(PQ) and D_(ST) could be fixed for each of up to 20 preset heart rateranges. In either case, it is envisioned that after the device has beenimplanted, the patient's physician would, through the programmer 68 ofFIG. 1, download from the cardiosaver 5 to the programmer 68, a recentelectrogram segment from the recent electrogram memory 472. Thephysician would then use the programmer 68 to select the values ofT_(PQ), T_(ST), D_(PQ) and D_(ST) for the heart rate in the downloadedrecent electrogram segment. The programmer 68 would then allow thephysician to choose to either manually specify the values of T_(PQ),T_(ST), D_(PQ) and D_(ST) for each heart rate range or have thecardiosaver 5 automatically adjust the values of T_(PQ), T_(ST), D_(PQ)and D_(ST) based on the R-R interval for each beat of any electrogramsegment collected in the future by the cardiosaver 5. It is alsoenvisioned that only the offset times, T_(PQ) and T_(ST), might beautomatically adjusted and the durations D_(PQ) and D_(ST) would befixed so that the average values of the ST and PQ segments V_(PQ) (512),V_(ST) (514), V′_(PQ) (512′) and V′_(ST) (514′) would always use thesame number of data samples for averaging.

[0190] While the simplest method of adjusting the times T_(PQ) andT_(ST) is to adjust them in proportion to the R-R interval from thepreceding R wave to the R wave of the current beat, a preferredembodiment of the present invention is to adjust the times T_(PQ) andT_(ST) in proportion to the square root of the R-R interval from thepreceding R wave to the R wave of the current beat. It is alsoenvisioned that a combination of linear and square root techniques couldbe used where T_(ST) and D_(ST) are proportional to the square root ofthe R-R interval while T_(PQ) and D_(PQ) are linearly proportional tothe R-R interval.

[0191] When used in pacemakers or combination pacemaker/ICDs itenvisioned that the start time T_(ST) and duration D_(ST) of the STsegment may have different values than during sinus rhythm (when thepacemaker is not pacing) as pacing the heart changes the characteristicsof ischemic ST shifts causing them to occur later relative to the startof the R wave. It is also envisioned, that the offset for the start ofthe ST segment may be better measured from the S Wave instead of the Rwave used for sinus rhythm when the pacemaker is not pacing. Thetechnique of using different timing parameters for start and durationwhen pacing can be applied to analysis of any sub-segment of theelectrogram including the sub-segment that includes the T wave peak.

[0192] Various techniques have been used to detect the R and S waves inelectrogram data. A well known technique is to look for a change inslope that exceeds a programmed threshold. Because the polarity of thewave depends on electrode placement in surface ECG, the slope thresholdis the same for both positive and negative slopes. Because the guardiansystem has the polarity in a right ventricle to implanted device fixed,the present invention envisions using different threshold values forpositive and negative slopes to better detect paced beats and/or PVCs.

[0193] The detection algorithm may need to differentiate between R, Sand T waves so as not to miscalculate the R-R interval between beats.This can be accomplished by measurement of the width of each of the R,S, and T waves where the R and S are always much narrower than the Twave. It is envisioned that the present invention would discriminate R(or S) vs. T wave by the width of the wave. For example, to be adetected R wave, the wave must have a width that is within a specifiedrange of the R waves that were measured within a pre-set time such as aminute in the past. In this way if the T wave spikes up during anischemic event it will be too wide to be considered an R wave and thedetection algorithm will not be fooled.

[0194] Another way to accomplish the same result is to use a separatehigh pass filter for the signal used for R wave detection where the Rwave detector high pass filter cuts has more low frequency attenuationthan the high pass filter used for the signal analyzed for ST segmentchanges. This technique is currently used in pacemakers and ICDs for Rwave detection but can also be applied to a stand alone cardiosaverdevice for ischemia detection. Typical high pass filter settings wouldbe as follows:

[0195] For R wave detection use a high pass filter with 6 dB attenuationat 10 Hz to 20 Hz.

[0196] For ST segment shift detection use a high pass filter with 6 dBattenuation at 0.1 to 0.5 Hz.

[0197] An example of a sequence of steps used to calculate the STdeviation 510 for the normal beat 500 are as follows:

[0198] 1. Identify the time T_(R) (509) for the peak of the R wave forthe beat 500,

[0199] 2. Calculate the time since the previous R wave and use that timeto look up or calculate the values of T_(PQ), T_(ST), D_(PQ) and D_(ST).

[0200] 3. Average the amplitude of the PQ segment 501 between the times(T_(R) −T_(PQ)) and (T_(R)−T_(PQ)+D_(PQ)) to create the PQ segmentaverage amplitude V_(PQ) (512),

[0201] 4. Average the amplitude of the ST segment 505 between the times(T_(R) +T_(ST)) and (T_(R)+T_(ST)+D_(ST)) to create the ST segmentaverage amplitude V_(ST) (514),

[0202] 5. Subtract V_(PQ) (512) from V_(ST) (514) to produce the STdeviation ΔV (510) for the beat 500.

[0203] Although only one normal beat 500 is shown here, there wouldtypically be multiple beats saved in the Y second long electrogramsegments stored in the recent electrogram memory 472 and the baselineelectrogram memory 474 of FIG. 4. At preset time intervals during theday step 458 of FIG. 5 will run the baseline parameter extractionsubroutine 440 that will calculate the “average baseline ST deviation”ΔV_(BASE) defined as the average of the ST deviations ΔV (510) for atleast two beats of a baseline electrogram segment. Typically the STdeviation of 4 to 8 beats of the baseline electrogram segment will beaveraged to produce the average baseline ST deviation ΔV_(BASE).

[0204] For each of “i” preset times during the day (at a time intervalof approximately 2W) an average baseline ST deviation ΔV_(BASE)(i) willbe calculated and saved in the calculated baseline data memory 475 forlater comparison with the ST deviation ΔV (510) of each beat of arecently collected electrogram. For example, in a preferred embodimentof the present invention, the average baseline ST deviation ΔV_(BASE)(i)is collected once an hour and there are be 24 values of ΔV_(BASE)(i)(ΔV_(BASE)(l), ΔV_(BASE)(2) . . . ΔV_(BASE)(24)) stored in thecalculated baseline data memory 475 of FIG. 4. An excessive ST shift fora single beat of a recently collected electrogram segment is thendetected when the ST deviation ΔV for that beat shifts by more than apredetermined threshold amplitude from the average baseline ST deviationΔV_(BASE)(i) collected approximately 24 hours before.

[0205] The ST shift of a given beat is calculated by subtracting theappropriate averaged baseline ST deviation ΔV_(BASE)(i) from the STdeviation ΔV for that beat. Assuming the R-R interval indicates that theheart rate for a beat is in the normal range then an excessive ST shiftfor a single beat is detected if (ΔV −ΔV_(BASE)(i)) is greater than thenormal ST shift threshold H_(normal) for the normal heart rate range.The heart signal processing program 450 of FIG. 5 requires that such anexcessive ST shift be positively identified in M out of N beats in threesuccessive recent electrogram segments before the alarm subroutine 490is activated. The threshold H_(normal) may be a fixed value that doesnot change over time and is set at the time of programming of thecardiosaver 5 with the programmer 68 of FIG. 1.

[0206] In a preferred embodiment, the threshold for detection ofexcessive ST shift is not fixed but is calculated as H_(ST)(i) from thei'th baseline electrogram segment stored in the baseline electrogrammemory 474 of FIG. 4. To do this the difference between the amplitude ofthe peak of the R wave V_(R) (503) and the average PQ segment amplitudeV_(PQ) (512) are calculated for each of at least 2 beats of eachbaseline electrogram segment by the baseline parameter extractionsubroutine 440. The average value AR(i) of this difference(V_(R)−V_(PQ)) for at least two beats of the i'th baseline electrogramsegment can be used to produce a threshold for ST shift detectionH_(ST)(i) that is proportional to the signal strength of the i'thbaseline electrogram segment. The advantage of this technique is that,if the signal strength of the electrogram changes slowly over time, thethreshold H_(ST)(i) for excessive ST shift detection will change inproportion.

[0207] The preferred embodiment of the present invention would have apreset percentage P_(ST) that is multiplied by ΔR(i) to obtain thethreshold H_(ST)(i) =P_(ST) X Δ_(R)(i). Thus, the threshold H_(ST)(i)would be a fixed percentage of the average height of the R wave peaksover the ST segments of the i'th baseline electrogram segment. Forexample, if P_(ST) is 25% an excessive ST shift on a given beat would bedetected if the ST shift (ΔV −ΔV_(BASE)(i)) is greater than thethreshold H_(ST)(i) where H_(ST)(i) is 25% of the average PQ to R heightΔR(i) of the i'th baseline electrogram segment.

[0208] In a preferred embodiment of the present invention heart signalprocessing program 450 of FIG. 5, the value X and Z are both 20 seconds,Y is 10 seconds, 2W is 60 minutes, U is 24 hours, W is 30 minutes, M is6 and N is 8. Therefore the steps 457 and 469 of FIG. 5 will check forexcessive ST shifts in 6 out of 8 beats from of the Y=10 second longelectrogram segment captured every 30 seconds as compared withparameters extracted from the baseline electrogram segment captured 24±½hour before. In this preferred embodiment baseline electrogram segmentsare captured once per hour.

[0209] It is also envisioned that the patient would undergo a stresstest following implant. The electrogram data collected by the implant 5would be transmitted to the programmer 68 of FIG. 1, and one or more ofthe parameters T_(PQ) (502), T_(ST) (504), D_(PQ) (506) and D_(ST) (508)of FIG. 6 would be automatically selected by the Programmer based on theelectrogram data from the stress test. The data from the stress testshould cover multiple heart rate ranges and would also be used by theprogrammer 68 to generate the excessive ST shift detection percentagethresholds P_(ST) for each of the heart rate ranges. In each case wherethe programmer 68 automatically selects parameters for the ST shiftdetection algorithm, a manual override would also be available to themedical practitioner. Such an override is of particular importance as itallows adjustment of the algorithm parameters to compensate for missedevents or false positive detections.

[0210] The S wave peak voltage V_(S) (507) is also shown on the baselinebeat 500 in FIG. 6. While the preferred embodiment of the presentinvention uses the average PQ to R wave amplitude ΔR(i) as thenormalization voltage for setting the threshold H_(ST)(i), it is alsoenvisioned that normalization voltage could be the average of the entireR wave to S wave amplitude (V_(R)−V_(S)) or it could be the larger ofΔR(i) or the PQ to S amplitude ΔS(i)=V_(S)−V_(PQ). It is important tonote here that the threshold H_(ST)(i) is set as a percentage of thebaseline average signal amplitude. This is important because thebaseline signal is only collected if the electrogram is normal andtherefore the thresholds would not be affected by transient changes insignal amplitude (e.g. R wave height) that can occur during an STelevation myocardial infarction. Therefore, for the purposes of thepresent invention the threshold H_(ST)(i) is calculated as a percentageof the average signal amplitude of at least two beats of the baselineelectrogram segment where the average signal amplitude of the baselinesegment can be any of the following:

[0211] the average PQ segment to R voltage difference ΔR(i),

[0212] the peak-to-peak voltage of the beat (i.e. the R to S wavevoltage difference) (V_(R)−V_(S)),

[0213] the average PQ segment to S wave voltage difference ΔS(i),

[0214] the larger of ΔR(i) or ΔS(i), or

[0215] any average signal amplitude calculated from at least two beatsof the baseline electrogram segment.

[0216]FIG. 7 illustrates a preferred embodiment of the baselineextraction subroutine 440. The subroutine 440 begins in step 439 bysaving in the i'th memory location in baseline electrogram memory 474 ofFIG. 4, the last Y second long electrogram segment saved into the“Recent” electrogram memory in step 454 of FIG. 5. This Y seconds ofelectrogram data then becomes the baseline electrogram segment forcalculating parameters for detection to be used during the 2 W longperiod of time U±W minutes in the future.

[0217] Next in step 441 the baseline extraction subroutine 440 finds theR wave peak times T_(R)(j) for the 1^(st) through (N+2)^(th) beat (j=1through N+2) in the baseline electrogram segment saved in step 439. Thisis a total of N+2 beats. Each time T_(R)(j) is typically counted fromthe beginning of the Y second long electrogram segment until the peak ofthe j'th R wave.

[0218] Next in step 442 the average R-R interval of the i'th baselineelectrogram segment RR(i) is calculated by averaging the R-R intervalsfor each of the N+1 beats (j=2 through N+2) where the R-R interval forbeat j is T_(R)(j)−T_(R)(j−1). For example, for beat 2, the R-R intervalis the time interval from the R wave peak of beat 1 (the very first Rwave) to the R wave peak of beat 2. I.e. R-R intervals before and aftereach of the N beats j=2 through j=N+1 are calculated. This step alsoidentifies any R-R intervals that are out of the “normal” range asdefined in the programming of the cardiosaver 5. In a preferredembodiment of the present invention, baseline data will only beextracted from “normal” beats. A normal beat is one in which the R-Rinterval both before and after the R wave is in the “normal range. Thisis a preferred technique to use as a too short R-R interval before the Rwave can affect the PQ segment amplitude and a too short R-R intervalafter the R wave can affect the ST segment amplitude, either of whichcould produce a false indication of excessive ST shift.

[0219] Next in step 443 the offsets T_(PQ), T_(ST), D_(PQ) and D_(ST)(see FIG. 6) are calculated. In one embodiment, T_(PQ) and T_(ST) arethe percentages φ_(PQ) and φ_(ST) multiplied by the average R-R intervalRR(i) respectively. This technique will adjust the location of the startof the PQ and ST segments to account for changes in heart rate. Thepercentages φ_(PQ) and φ_(ST) would be selected by the patient's doctorbased on “normal” electrogram segments analyzed by the programmer 68 ofFIG. 1. Another embodiment of the present invention uses fixed timeoffsets T_(PQ) and T_(ST) that are programmed by the patient's doctor.Similarly the duration of the PQ and ST segments D_(PQ) and D_(ST) (seeFIG. 6) can be calculated by multiplying the percentages δ_(PQ) andδ_(ST) times the average R-R interval RR(i) respectively. Thepercentages δ_(PQ) and δ_(ST) would also be selected by the patient'sdoctor using the programmer 68. The preferred embodiment of the presentinvention uses fixed segment durations D_(PQ) and D_(ST) that areprogrammed by the patient's doctor. Using fixed durations D_(PQ) andD_(ST) has the advantage of keeping the same number of samples averagedin each calculation of the average PQ and ST segment amplitudes V_(PQ)and V_(ST) respectively.

[0220] Next in step 444 for each of the N beats (j=2 through N+1)identified by step 422 as a normal beat, V_(PQ)(j) the average of the PQsegment amplitude of the j'th beat over the duration D_(PQ) beginningT_(PQ) before the peak T_(R)(j) and V_(ST)(j) the average ST segmentamplitude of the j'th beat over the duration D_(ST) beginning T_(ST)after the time T_(R)(j) are calculated. Similarly, step 444 calculatesthe peak T wave heights V_(T)(j).

[0221] For each beat the ST deviation ΔV_(ST) (j) that is the differencebetween V_(ST)(j) and V_(PQ)(j) is then calculated in step 445.Similarly, step 445 calculates the T wave deviation ΔV_(T) (j) that isthe difference between V_(T)(j) and V_(PQ)(j). It should be noted thatstep 455 of FIG. 5 will only allow the baseline extraction subroutine tobe run if less than 2 too short beats are present, thus at least N−2 ofthe N beats used for baseline data extraction will be normal beats.Although there is a limit here of less than 2 short beats, it isenvisioned that other minimum numbers of short beats than 2 might alsobe used.

[0222] Next in step 446 the ST deviation ΔV_(ST)(j) for all normal beatswithin the N beats is averaged to produce the i'th average baseline STdeviation ΔV_(BASE)(i). Similarly, in step 446 the T wave deviationΔV_(T)(j) for all normal beats within the N beats is averaged to producethe i'th average baseline T wave deviation ΔT_(BASE)(i).

[0223] An alternate embodiment of the present invention would also checkfor excessive ST shift on each normal beat and exclude any such beatsfrom the average baseline ST deviation and T wave deviationcalculations.

[0224] Next in step 447, ΔR(i) the average of the height of the peak ofthe j'th R wave above the average PQ segment V_(PQ)(j) is calculated forthe normal beats. ΔR(i) acts as an indication of the average signalstrength of the i'th baseline electrogram segment. ΔR(i) is used toprovide a detection threshold for excessive ST shift that will adapt toslow changes in electrogram signal strength over time. This is of mostvalue following implant as the sensitivity of the electrodes 14 and 17may change as the implant site heals.

[0225] ΔT_(BASE) (i) can either be the average of the signal samples ofthe entire T waves or it can be the average of the peak amplitude of theT waves in the normal beats. It is also envisioned, that if both ST andT wave shift detection are used, a cardiac event could be declared ifeither excessive ST shift or T wave shift detects a change (this ispreferred) or the program could require that both excessive ST shift andT wave shift be present.

[0226] Next in step 448, the threshold for ST shift detection for normalheart rates H_(ST)(i) is calculated by multiplying the programmedthreshold percentage P_(ST) of ΔR(i). Also in step 448, if the T waveshift detector is being used, the threshold for T wave shift detectionfor normal heart rates H_(T)(i) is calculated by multiplying theprogrammed threshold percentage P_(T) of ΔR(i).

[0227] Finally in step 449, the extracted baseline parametersΔV_(BASE)(i), ΔT_(BASE)(i), ΔR(i), H_(ST)(i) and H_(T)(i) are saved tothe calculated baseline data memory 475. The baseline extractionsubroutine 440 has ended and the program returns to the main heartsignal processing program 450 step 451 of FIG. 5.

[0228] One embodiment of ST shift and T wave shift detection might use abaseline for ST shift detection that is 24±½ hour before and a baselinefor T wave shift that is 1 to 4 minutes in the past. This would requirethat the baseline extraction subroutine 440 be run for T wave shiftparameters approximately every 60 seconds and for ST segment parametersevery hour.

[0229] Although the baseline extraction subroutine 440 is described hereas using the same “N” as the number of beats processed as the ST shiftdetection steps 457 and 469 of FIG. 5, it is envisioned that either agreater or lesser number of beats could be used for baseline extractionas compared with the number of beats “N” checked for excessive ST shiftsin FIG. 5.

[0230] Typical values used for the baseline extraction subroutine 440 asshown in FIG. 7 would be N=8 to average the data over 8 beats usingbeats 2 through 9 of the Y second long electrogram segment. However, itis envisioned that as few as 1 beat or as many as 100 beats or highercould be used to calculate the parameters extracted by subroutine 440.Also even though the preferred embodiment of the present inventionextracts baseline data only from “normal” beats, it is envisioned thatusing all 8 beats would usually yield an acceptable result.

[0231] Although the baseline extraction subroutine 440 shows theextraction of parameters for identifying excessive ST shifts and T waveshifts, the cardiosaver 5 would function with either of these detectionmethods or could use other techniques to measure the changes inelectrogram signals indicating one or more coronary event.

[0232]FIG. 8 illustrates a preferred embodiment of the alarm subroutine490. The alarm subroutine 490 is run when there have been a sufficientnumber of events detected to warrant a major event cardiac alarm to thepatient. The alarm subroutine 490 begins with step 491 where the entirecontents of both baseline electrogram memory 474 and recent electrogrammemory 472 of FIG. 4 are saved into the event memory 476. This saves theabove mentioned electrogram data in a place where it is not overwrittenby new baseline or recent electrogram data to allow the patient'sphysician to review the electrogram segments collected during a periodof time that occurred before the alarm. In a preferred embodiment with24 baseline electrogram segments collected once per hour, and 8 recentelectrogram segments collected every 30 seconds, the physician will beable to review a significant amount of electrogram data from the 4minutes just before the cardiac event as well as being able to see anychanges in the 24 hours before the event.

[0233] Next; in step 492 the internal alarm signal is turned on byhaving the CPU 44 of FIG. 4 cause the alarm sub-system 48 to activate amajor event alarm signal.

[0234] Next in step 493 the alarm subroutine instructs the CPU 44 tosend a major event alarm message to the external alarm system 60 of FIG.1 through the telemetry sub-system 46 and antenna 35 of the cardiosaver5 of FIG. 4. The alarm message is sent once every L1 seconds for L2minutes. During this time step 494 waits for an acknowledgement that theexternal alarm has received the alarm message. After L2 minutes, if noacknowledgement is received, the cardiosaver 5 of FIG. 1 gives up tryingto contact the external alarm system 60. If an acknowledgement isreceived before L2 minutes, step 495 transmits alarm related data to theexternal alarm system. This alarm related data would typically includethe cause of the. alarm, baseline and last event electrogram segmentsand the time at which the cardiac event was detected.

[0235] Next in step 496, the cardiosaver 5 transmits to the externalalarm system 60 of FIG. 1 other data selected by the patient's physicianusing the programmer 69 during programming of the cardiosaver. Thesedata may include the detection thresholds H_(ST)(i), H_(T)(i) and otherparameters and electrogram segments stored in the cardiosaver memory 47.

[0236] Once the internal alarm signal has been activated by step 492, itwill stay on until the clock/timing sub-system 49 of FIG. 4 indicatesthat a preset time interval of L3 minutes has elapsed or the cardiosaver5 receives a signal from the external alarm system 60 of FIG. 1requesting the alarm be turned off.

[0237] To save power in the implantable cardiosaver 5, step 496 mightcheck once every minute for the turn off signal from the external alarmsystem 60 while the external alarm system 60 would transmit the signalcontinuously for slightly more than a minute so that it will not bemissed. It is also envisioned that when the alarm is sent to theexternal alarm system 60, the internal clock 49 of the cardiosaver 5 andthe external alarm system 60 can be synchronized so that the programmingin the external alarm system 60 will know when to the second, that thecardiosaver will be looking for the turn off signal.

[0238] At this point in the alarm subroutine 490 step 497 begins torecord and save to event memory 476 of FIG. 4, an E second longelectrogram segment every F seconds for G hours, to allow the patient'sphysician and/or emergency room medical professional to read out thepatient's electrogram over time following the events that triggered thealarm. This is of particular significance if the patient, his caregiveror paramedic injects a thrombolytic or anti-platelet drug to attempt torelieve the blood clot causing the acute myocardial infarction. Byexamining the data following the injection, the effect on the patientcan be noted and appropriate further treatment prescribed.

[0239] In step 498 the alarm subroutine will then wait until a resetsignal is received from the physician's programmer 68 or the patientoperated initiator 55 of the external alarm system 60 of FIG. 1. Thereset signal would typically be given after the event memory 476 of FIG.4 has been transferred to a component of the external equipment 7 ofFIG. 1. The reset signal will clear the event memory 476 (step 499) andrestart the main program 450 at step 451.

[0240] If no reset signal is received in L6 hours, then the alarmsubroutine 490 returns to step 451 of FIG. 5 and the cardiosaver 5 willonce again begin processing electrogram segments to detect a cardiacevent. If another event is then detected, the section of event memory476 used for saving post-event electrogram data would be overwrittenwith the pre-event electrogram data from the new event. This processwill continue until all event memory is used. I.e. it is more importantto see the electrogram data leading up to an event than the datafollowing detection.

[0241]FIG. 9 illustrates the function of the hi/low heart ratesubroutine 420. The hi/low heart rate subroutine is meant to run whenthe patient's heart rate is below the normal range (e.g. 50 to 80 beatsper minute) or above the elevated range that can occur during exercise(e.g. 80 to 140 beats per minute). A low heart rate (bradycardia) mayindicate the need for a pacemaker and should prompt a SEE DOCTOR alertto the patient if it does not go away after a programmed period of time.Very high heart rate can be indicative of tachycardia or ventricularfibrillation and is serious if it does not quickly go away and shouldwarrant a major event alarm like a detected AMI.

[0242] The hi/low heart rate subroutine 420 begins with step 421 wherethe electrogram segment of Y seconds collected in steps 453 and 454 ofFIG. 5 is saved to the event memory 476 (step 421) because the patient'sdoctor may wish to know that the high or low heart rate occurred. Oncethe Y second long electrogram segment is saved, step 422 of the hi/lowheart rate subroutine 420 directs the processing in different directionsdepending on if the heart rate is too high, too low or unsteady. Ifunsteady, the unsteady heart rate subroutine 410 illustrated in FIG. 12is run. If it is too high, step 423 increments the event counter k by 1,then step 424 checks whether the event counter k is equal to 3. Althoughthis embodiment uses k=3 events as the trigger to run the alarmsubroutine 490 it is envisioned that k=1 or 2 or k values higher than 3can also be used.

[0243] In step 424, If k=3 then the alarm subroutine 490 illustrated inFIG. 8 is run. If k less than 3 then in step 425 the hi/low heart ratesubroutine 420 waits for “B” seconds and checks again in step 426 if theheart rate is still too high. If the heart rate is still too high, thehi/low heart rate subroutine 420 returns to step 423 where the eventcounter is incremented by 1. If the heart rate remains high, the hi/lowheart rate subroutine 420 will loop until k is equal to 3 and the alarmsubroutine 490 is run. If the heart rate does not remain high in step426, the hi/low heart rate subroutine 420 will return to step 453 of themain heart signal processing program 450 illustrated in FIG. 5. ST shiftamplitude (and/or T wave shift) is not checked during the high heartrate section of the hi/low heart rate subroutine 420 as the presence ofa very high heart rate could alter the detection of changes in ST and PQsegments of the electrogram giving false indications. Very high heartrate is, by itself, extremely dangerous to the patient and is thereforea major cardiac event.

[0244] If in step 422, the heart rate is too low rather than too high,the hi/low heart rate subroutine 420 will proceed to step 431 where theY second long electrogram segment is checked for an excessive ST shiftin the same way as step 457 of the main heart signal processing program450 illustrated in FIG. 5. In other words, the ST deviation on M out ofN beats must be shifted at least H_(ST)(i) from the baseline average STdeviation ΔV_(BASE)(i) of the i'th baseline electrogram segment. Ifthere is a detected excessive ST shift in step 431, the hi/low heartrate subroutine 420 returns to run the ST shift verification subroutine460 illustrated in FIG. 5. As with step 457 of the main heart signalprocessing program 450, the detection of M−N+1 OK beats withoutexcessive ST shift is sufficient for a negative detection and theprogram can then proceed on to step 432.

[0245] If there is not an excessive ST shift detected in step 431, step432 causes the hi/low heart rate subroutine 420 in step 432 to wait for“C” seconds then buffer and save a new Y second long electrogram segmentas in steps 453 and 454 of the main heart signal processing program 450of FIG. 5. Once the new Y second long electrogram segment is collected,the hi/low heart rate subroutine 420 checks in step 433 if the heartrate is still too low. If it is no longer too low, the system returns tostep 455 of the main heart signal processing program 450 illustrated inFIG. 5.

[0246] If the heart rate remains too low, then step 434 checks for anexcessive ST shift as in step 431. If there is an excessive ST shift instep 434, the hi/low heart rate subroutine 420 returns to run the STshift verification subroutine 460 of FIG. 5. If there is not anexcessive ST shift detected in step 434, step 435 causes the hi/lowheart rate subroutine 420 in step 435 to wait for another “C” secondsthen buffer and save another Y second long electrogram segment as insteps 453 and 454 of the main heart signal processing program 450 ofFIG. 5. Once this Y second long electrogram segment is collected, thehi/low heart rate subroutine 420 checks in step 436 if the heart rate isstill too low (for the 3^(rd) time). If it is no longer too low, thesystem returns to step 455 of the main heart signal processing program450 of FIG. 5. If the heart rate remains too low, then step 437 checksfor an excessive ST shift as in steps 431 and 434. If there is anexcessive ST shift in step 437, the hi/low heart rate subroutine 420returns to run the ST shift verification subroutine 460 of FIG. 5. Ifthere is not an excessive ST shift detected in step 437, the step 438saves the contents of the most recently collected Y second longelectrogram segment and the to the event memory 476 for later review bythe patient's doctor.

[0247] If the hi/low heart rate subroutine 420 reaches step 438 then thepatient's heart rate has been too low even after two waits of “C”seconds. Now the hi/low heart rate subroutine 420 proceeds to step 427to turn on the internal “SEE DOCTOR” alarm signal. Step 427 also sendsout to the external alarm system 60 of FIG. 1, a signal to activate the“SEE DOCTOR” alarm signal of the external alarm system 60 that mayinclude a text or played speech message indicating the cause of thealarm. E.G. the external alarm system speaker 57 of FIG. 1 could emitwarning tones and a text message could be displayed or the speaker 57might emit a spoken warning message to the patient.

[0248] Note that during the checking for continued low heart rate, STshift amplitudes are still checked after each wait because it is wellknown that low heart rate can be a byproduct of an acute myocardialinfarction.

[0249] Finally in step 428, the hi/low heart rate subroutine 420 willkeep the “SEE DOCTOR” alarm signal turned on for L4 minutes or untilreceipt of a signal from the external alarm system 60 to turn off thealarm signal. After the “SEE DOCTOR ALERT signal is enabled, the lowheart rate limit, below which the hi/low heart rate subroutine 420 isrun, is changed by step 429 to be just below the average heart ratemeasured in step 436. Once the patient is warned to go see the doctor,additional warnings will be annoying and therefore the low rate limit isbest changed. This allows the hi/low heart rate subroutine 420 to thenreturn to step 452 of the main program where it will continue to monitorST shift amplitudes to provide early detection of acute myocardialinfarction. Actual programming of the cardiosaver 5 may use R-R intervalinstead of heart rate and it is understood that either is sufficient andone can be easily computed from the other.

[0250] Although steps 431, 434 and 437 indicate the subroutine 420 is tolook for an ST shift, other ischemia indications such as T wave spiking,either alone or in combination with ST shift detection may be used. Alsoin steps 431, 434 and 437 if no shift is detected, the event counter kis reset to 0 if it is not already 0.

[0251]FIG. 10 illustrates the ischemia subroutine 480 that providesdecision making for the cardiosaver 5 in the event of an elevated heartrate such as that would occur during exercise by the patient. Theischemia subroutine 480 uses a beat counter j to indicate the beatwithin a Y second long electrogram segment. A beat is defined as asub-segment containing exactly one R wave of the Y second longelectrogram segment. The ischemia subroutine 480 begins in step 481 byinitializing the beat counter j to a value of 2. Then in step 482, theR-R interval range A for the beat j is determined. For example thatthere could be between 4 R-R interval ranges A=1 to 4 of 750 to 670, 670to 600, 600 to 500 and 500 to 430 milliseconds respectively. These wouldcorrespond to heart rate intervals of 80 to 90, 90 to 100, 100 to 120and 120 to 140 beats per minute. The number of ranges A and the upperand lower limit of each range would be programmable by the patient'sphysician from the programmer 68 of FIG. 1.

[0252] Next in step 483 the programmed ischemia multiplier μ(A) isretrieved from the programmable parameters 471 of FIG. 4. μ(A) is theallowable factor increase or decrease in ST shift detection thresholdfor the R-R interval range A. In other words, because the patient mayhave some ischemia during elevated heart rates from exercise, thepatient's physician can program μ(A)s that are greater than 1 and mightincrease with each successive heart rate range. For example, if the R-Rinterval ranges are 750 to 670, 670 to 600, 600 to 500 and 500 to 430milliseconds the corresponding μ(A)s might be 1.1, 1.2, 1.3 and 1.5.This would require that the ST shift in the R-R interval range of A=4(500 to 430 milliseconds) be one and a half times as large as duringnormal heart rates in order to qualify as a cardiac event. It isenvisioned that the patient could undergo an exercise stress test at atime after implant when the implanted leads have healed into the wall ofthe heart and electrogram segments captured by the cardiosaver 5 duringthat stress test would be reviewed by the patient's physician todetermine the appropriate range intervals and ischemia multipliers tohelp identify a worsening of the patient's exercise induced ischemiafrom the time when the stress test is conducted.

[0253] It is also envisioned that in order to detect smaller changes invessel narrowing than a full acute myocardial infarction, thecardiosaver S of FIGS. 1-4 might use μ(A)s that are less than one. Forexample, if the R-R interval ranges are 750 to 670, 670 to 600, 600 to500 and 500 to 430 milliseconds the corresponding μ(A)s might be 0.5,0.6, 0.7 and 0.8. Thus in this example, in the R-R interval range of 750to 670 milliseconds, the threshold for ischemia detection would be halfof what it is for the normal heart rate range.

[0254] Once the ischemia multiplier has been retrieved, step 484calculates the ischemia ST shift threshold θ(A) for the R-R intervalrange A where θ(A)=H_(ST)(i)×μ(A) where H_(ST)(i) is the current STshift threshold for normal heart rates. Next in step 485, the ischemiasubroutine 480 checks if for the beat j the ST shift is greater than theischemia threshold θ(A). If it is not greater, step 487 then checks ifthe N'th beat has been examined. If the ST shift of the j'th beatexceeds the ischemia threshold θ(A) then step 486 checks if M beats withST shifts greater than θ(A) have been seen. If they have not been seenproceed to step 487. If in step 487, the Nth beat has been examined,return to step 451 of the main heart signal processing program 450 ofFIG. 5. If N beats have not yet been examined, increment j by 1 in step489 and loop back to step 482.

[0255] If M beats with excessive ST shift are found by step 486, step581 saves the current Y second long electrogram segment to the EventMemory 476, then in step 582 the event counter k is incremented by 1followed by step 583 checking if k is equal to 3. If k is less than 3then the ischemia subroutine 480 continues by sleeping for Z seconds instep 584, then buffering a new Y second long electrogram segment in step585, saving in step 586 the new Y second long electrogram segment to thenext location in recent electrogram memory 472 of FIG. 4. and thenchecking if the heart rate is still elevated in step 587. If the heartrate is still elevated in step 587, the loop checking for ischemia isrun again starting with step 481. If the heart rate is no longerelevated then step 588 checks if the heart rate is too high, too low orunsteady. If such is the case, the hi/low heart rate subroutine 420 isrun. If the heart rate is not high, low or unsteady, the ischemiasubroutine 480 ends and the program returns to step 469 of the ST shiftverification subroutine 460 of FIG. 5. This will allow an excessive STshift detected at elevated heart rate that stays shifted when the heartrate returns to normal to quickly trigger the AMI alarm. This worksbecause k is either 1 or 2 at this point so either 2 or 1 more detectionof excessive ST shift with normal heart rate will cause a major eventAMI alarm. If however k=3 in step 582, then the last detection ofexcessive ST shift occurred during an elevated heart rate and will betreated as exercise induced ischemia rather than an acute myocardialinfarction.

[0256] So if k=3 (i.e. exercise induced ischemia has been detected) instep 582 the ischemia subroutine 480 moves on to step 681 where itchecks if it has been more than L5 minutes since the first time thatexercise induced ischemia was detected where k=3 in step 583.

[0257] If it has been less than L5 minutes since the first detection ofexercise induced ischemia then the internal SEE DOCTOR ALERT signal isturned on by step 682 if it has not already been activated.

[0258] If it has been more than L5 minutes, then the alarm subroutine490 is run. This will change the SEE DOCTOR ALERT signal previouslystarted in step 682 to a major event AMI alarm if the excessive ST shiftat an elevated heart rate does not go away within L5 minutes. Similarly,if the patient stops exercising and his heart rate returns to normal butthe excessive ST shift remains, then the alarm subroutine 490 will alsobe run.

[0259] If it has been less than L5 minutes and the SEE DOCTOR alertsignal has not been already been activated, step 683 next sends amessage to the external alarm system 60 of FIG. 1 to activate the SEEDOCTOR external alarm signal and indicate to the patient by a text ofspoken message that he should stop whatever he is doing, and sit or liedown to get his heart rate to return to normal. Following this, in step684 the ischemia subroutine 480 will keep the SEE DOCTOR ALERT signal onfor L4 minutes from the first time it is turned on or until the receiptof an off signal from the alarm disable button 59 of the external alarmsystem 60 of FIG. 1. The program then returns to step 451 of the mainprogram 451 of FIG. 5 to continue to examine the patient's heartsignals.

[0260]FIG. 11 diagrams the alarm conditions 600 that are examples of thecombinations of major and minor events that can trigger an internalalarm signal (and/or external alarm signal for the guardian system ofFIG. 1. Box 610 shows the combinations 611 through 617 of major cardiacevents that can cause the alarm subroutine 490 to be run. These includethe following:

[0261] 611. 3 ST shift events (detections of excessive ST shift) witheither a normal heart rate or a low heart rate.

[0262] 612. 2 ST shift events with a normal or low heart rate and 1event from heart rate too high.

[0263] 613. 1 ST shift event with a normal or low heart rate and 2events from heart rate too high.

[0264] 614. 3 events from heart rate too high.

[0265] 615. 3 ST shift events with either a normal, low or elevatedheart rate (ischemia) where the last detection is at a normal or lowheart rate.

[0266] 616. 3 events (excessive ST shift or high heart rate) where thelast event is high heart rate.

[0267] 617. An ischemia alarm indication from conditions in box 620 thatremains for more than L5 minutes after the first detection of ischemia.

[0268] The ischemia alarm conditions 620 include:

[0269] 621. 3 ST shift events with either a normal, low or elevatedheart rate (ischemia) where the last detection is at an elevated heartrate.

[0270] 622. Any 3 events including a too high heart rate event where thelast detection is an excessive ST shift at an elevated heart rate.

[0271] If either of the ischemia alarm conditions 620 is met and it isless than L5 minutes since the exercise induced ischemia was firstdetected, then the SEE DOCTOR ALERT signal will be turned on by step 682of the ischemia subroutine 480 if it has not already been activated.

[0272] Box 630 shows the other minor event alarm conditions includingthe bradycardia alarm condition 632 that is three successive electrogramsegments collected with heart rate too low and the unsteady heart ratealarm condition 635 that is caused by more than P_(unsteady)% of beatshaving a too short R-R interval. If here are too many (as programmed bythe doctor) consecutive electrogram segments with insufficient normalbeats 637 to be able to process for cardiac event detection, theprogramming may need modification or there is something else wrong.These will trigger the SEE DOCTOR alert signal initiated by step 427 ofthe hi/low heart rate subroutine 420 for the bradycardia alarm condition632 and step 416 of the unsteady hart rate subroutine 410 for theunsteady heart rate alarm condition 635. Also triggering the SEE DOCTORalert signal is a low battery condition 636.

[0273]FIG. 12 is a block diagram illustrating the unsteady heart ratesubroutine 410. The subroutine 410 is run if the R-R interval variesgreatly over many of the beats in the Y second long electrogram segmentcollected by steps 453 and 454 of the main heart signal processingprogram 450. As previously described, one technique for identifying suchan unsteady heart rate is to compare the two shortest R-R intervals andthe 2 longest intervals. If the difference between the both of the twoshortest and the average of the two longest R-R intervals are more thana programmed percentage a, an unsteady heart rate is identified. Forexample the programmed percentage α might be 25% so that if the twoshortest R-R intervals are each more than 25% less than the average ofthe two longest R-R intervals, then the heart rate is unsteady. It isenvisioned that if a longer time Y is used for electrogram segmentcollection then it might require 3 or more “short ” beats to indicatedan unsteady heart rate. If there is zero or one short beat, the mainheart signal processing program 450 will move on to step 456 havingmarked all of the “normal” beats in the Y second long electrogramsegment. A normal beat is defined as a beat including where the R-Rintervals before and after the R wave are both in the normal range (i.e.not too short).

[0274] The unsteady heart rate subroutine 410 begins in step 411 bychecking for at least N normal beats in the most recently collectedelectrogram data. When the subroutine begins there is only one Y secondlong electrogram segment being examined. If there are not N normalbeats, then the subroutine 410 will wait X seconds in step 419 before anadditional Y second long electrogram segment is collected in step 412after the . Step 411 then will check for N normal beats in the two Ysecond long electrogram segments (i.e. 2Y seconds of electrogram data).This loop of steps 411 and 412, where each time Y additional seconds ofelectrogram is collected, will continue until N normal beats are found.

[0275] It is envisioned that step 411 could also check for beats withelevated heart rate R-R intervals or might include elevated heart ratebeats as “normal” beats by expanding the allowed range of the R-Rinterval for a normal beat. Once N“normal” beats are found by step 411,then step 413 checks for an excessive ST shift in M out of the N normalbeats similar to step 457 of FIG. 5. Step 413 could also (as in step 457of FIG. 5) look for an excessive T wave shift. If an excessive ST shift(and/or T wave shift) is detected by step 413, the program returns tothe ST shift verification subroutine 460 of FIG. 5.

[0276] If excessive ST shift (and/or T wave shift) are not detected bystep 413, then step 414A checks if more than P_(unsteady)% of all thebeats (not just the normal beats) in the electrogram data collected havea too short R-R interval as defined above by the programmed parameter a.If not the program returns to step 451 of the main heart signalprocessing program 450 of FIG. 5. If, however, more than P_(unsteady)%of the beats have a short R-R interval, then step 414B ascertains ifthere have been N_(u) sequential electrogram segments having more thanP_(unsteady)% of the beats with short R-R intervals. If the number isless than Nu then this then the program returns to step 451 of the mainheart signal processing program 450 of FIG. 5. If the number is Nu thenstep 415 saves all the current electrogram data to event memory 476 ofFIG. 4 and step 416 turns on the SEE DOCTOR alert signal with theinternal alarm sub-system 48 of FIG. 4 and also initiates an externalalarm signal by the external alarm system 60 of FIG. 1 with a text orspoken message to the patient indicating that the SEE DOCTOR alertsignal is the result of detection of unsteady heart rate. As in the caseof other SEE DOCTOR alert signals, step 417 will keep the “See Doctor”alarm mechanism turned on for L4 minutes from the first detection ofunsteady heart rate or until receipt of a signal from the external alarmsystem 60 to turn off the alarm. To avoid continuously alarming thepatient, once the SEE DOCTOR alert has sounded, the system will wait fora preset time programmed by the patient's physician before allowingreactivation of the SEE DOCTOR ALERT. Alternately, there may be adefault wait period such as 12 hours or 1 day or the system may beprogrammed to only sound the SEE DOCTOR alert once for each indicationuntil reset by the physician's programmer.

[0277]FIG. 13 shows a modified embodiment of the guardian system 510.The cardiosaver implant 505 with lead 512, electrode 514, antenna 516,header 520 and metal case 511 would be implanted subcutaneously in apatient at risk of having a serious cardiac event such as an acutemyocardial infarction. The lead 512 could be placed eithersubcutaneously or into the patient's heart. The case 511 would act asthe indifferent electrode. The system 510 also included externalequipment that includes a physician's programmer 510 an external alarmtransceiver 560 and a pocket PC 540 with charger 566. The external alarmtransceiver 560 has its own battery 561 and includes an alarm disablebutton 562 radiofrequency transceiver 563, speaker 564, antenna 565 andstandard interface card 552. The cardiosaver 505 has the samecapabilities as the cardiosaver 5 of FIGS. 1 through 4.

[0278] The standardized interface card 552 of the external alarmtransceiver 510 can be inserted into a standardized interface card slotin a handheld or laptop computer. The pocket PC 540 is such a handheldcomputer. The physician's programmer 510 is typically a laptop computer.Such standardized card slots include compact flash card slots, PCMCIAadapter (PC adapter) card slots, memory stick card slots, Secure Digital(SD) card slots and Multi-Media card slots. The external alarmtransceiver 510 is designed to operate by itself as a self-containedexternal alarm system, however when inserted into the standardized cardslot in the pocket PC 540, the combination forms an external alarmsystem with enhanced functionality. For example, in stand alone modewithout the pocket PC 540, the external alarm transceiver 560 canreceive alarm notifications from the cardiosaver implant 505 and canproduce an external alarm signal by generating one or more soundsthrough the speaker 564. These sounds can wake the patient up or provideadditional alerting to that provided by the internal alarm signalgenerated by the cardiosaver 505. The alarm disable button 562 canacknowledge and turn off both external and internal alarm signals. Thestandalone external alarm transceiver 560 therefore provides keyfunctionality could be small enough to wear on a chain around the neckor on a belt.

[0279] When plugged into the pocket PC 540, the external alarmtransceiver 560 can facilitate the display of text messages to thepatient and electrogram data that is transmitted from the cardiosaver505. The pocket PC 540 also enables the patient operated initiator 55and panic button 52 capabilities of the external alarm system 60 ofFIG. 1. Being a pocket PC also readily allows connection to wirelesscommunication capabilities such as wireless internet access that willfacilitate retransmission of data to a medical practitioner at ageographically remote location. It is also envisioned that the charger566 could recharge the batter 551 when the external alarm adaptor 560 isplugged into the pocket PC 540.

[0280] The external alarm transceiver 560 can also serve as the wirelesstwo-way communications interface between the cardiosaver 505 and theprogrammer 510. The physician's programmer 510 is typically a laptopcomputer running some version of the Microsoft Windows operating system.As such, any or the above standardized slot interfaces can be eitherdirectly interfaced to such a laptop computer or interfaced using areadily available conversion adaptor. For example, almost all laptopcomputers have a PCMCIA slot and PCMCIA card adaptors are available forcompact flash cards, Secure Digital cards etc. Thus the external alarmadaptor 560 could provide the interface to the physician's programmer510. This provides additional security as each cardiosaver implant 505and external alarm adaptor 560 could be uniquely paired with built insecurity codes so that to program the implant 505, the physician wouldneed the patient's external alarm adaptor 560 that would act both as awireless transceiver and as a security key.

[0281] Although the guardian system 10 as described herein could clearlyoperate as a stand-alone system, it is clearly conceivable to utilizethe guardian system 10 with additional pacemaker or implanteddefibrillator circuitry. As shown in FIG. 4, pacemaker circuitry 170and/or defibrillator circuitry 180 could be made part of any cardiosaver5 or 505. Furthermore, two separate devices (one pacemaker or onedefibrillator plus one cardiosaver 5) could be implanted within the samepatient.

[0282]FIG. 14 illustrates a preferred physical embodiment of theexternal alarm transceiver 560 having standardized interface card 552,alarm disable button 562 labeled “ALARM OFF” and speaker 564. It is alsoenvisioned that by depressing and holding the alarm disable button 562for a minimum length of time, when there is not an alarm, the externalalarm transceiver could verify the operational status of the cardiosaver505 and emit a confirming sound from the speaker 564.

[0283]FIG. 15 illustrates the physical embodiment of the combinedexternal alarm transceiver 560 and pocket PC 540 where the standardizedinterface card 552 has been inserted into a matching standardizedinterface card slot the pocket PC 540. The screen 542 of the pocket PC540 shows an example of the display produced by an external alarm systemfollowing the detection of an acute myocardial infarction by thecardiosaver 505. The screen 542 of FIG. 15 displays the time of thealarm, the recent electrogram segment from which the cardiac event wasdetected and the baseline electrogram segment used for comparison in thecardiac event detection. Such a display would greatly facilitatediagnosis of the patient's condition upon arrival at an emergency roomand could eliminate the need for additional electrocardiogrammeasurements before the patient is treated.

[0284]FIG. 16 shows and advanced embodiment of the external alarmtransceiver 720 having a battery 721, an alarm disable button 722, a RFtransceiver for data communication to and from the implanted device, aloudspeaker 724, a microphone 727, a local area wireless interface 723,a standard interface 728 and a long distance (LD) voice/datacommunication interface 729. The function of the alarm disable button722 and the radiofrequency transceiver 723 are as described for thesimilar devices shown in FIG. 13.

[0285] The local area wireless interface 723 provides wirelesscommunication within a building (e.g. home, doctor's office or hospital)to and from the implant 505 with lead 512 and antenna 516 through theexternal alarm transceiver 720 from and to assorted external equipmentsuch as

[0286] Pocket PCs 702, Palm OS PDAs, Notebook PCs, physician'sprogrammers 704 and tablet diagnostic systems 706. The means fortransmission from the local area wireless interface 723 may be byradiofrequency or infra-red transmission. A preferred embodiment of thelocal area wireless interface 723 would use a standardized protocol suchas IRDA with infra-red transmission and Bluetooth or WiFi (802.11.a, b,or g) with radiofrequency transmission. The local area wirelessinterface 723 would allow display of implant data and the sending ofcommands to the implant 505.

[0287] The standard interface 728 provides a physical (wired) connectionfor data communication with devices nearby to the patient for thepurposes of displaying data captured by the implant 505 and for sendingcommands and programs to the implant 505. The standard interface 728could be any standard computer interface; for example: USB, RS-232 orparallel data interfaces. The pocket PC 702 and physician's programmer704 would have functionality similar to the pocket PC 540 andphysician's programmer 510 of FIG. 13.

[0288] The tablet diagnostic system 706 would provide a level offunctionality between that of the pocket PC 702 and physician'sprogrammer 706. For example, the tablet diagnostic system would have theprogrammer's ability to download complete data sets from the implant 505while the pocket PC is limited to alarm and baseline electrogramsegments or the most recent electrogram segment. The tablet diagnosticsystem 706 would be ideal for an emergency room to allow emergency roommedical professionals to quickly view the electrogram data stored withinthe implant 505 to assess the patient's condition. The recentlyintroduced Tablet PCs such as the Toshiba Portege 3500 or the CompaqTC1000 have IRDA, WiFi and USB interfaces built into them and so wouldmake an ideal platform for the tablet diagnostic system 706. It isenvisioned that such a tablet diagnostic system in an emergency room ormedical clinic would preferably be connected to its own external alarmtransceiver. The tablet diagnostic system 706 could be hand held ormounted on a wall or patient bed. A unit located near the bed of anincoming patient having a guardian implant 505 would enable display ofpatient diagnostic data without requiring any attachments to thepatient. Such wireless diagnosis is similar to that envisioned for thetricorder and diagnostic beds of the Star Trek science fiction seriescreated by Gene Roddenberry.

[0289] The long distance voice/data communication interface 729 withmicrophone 727 and also attached to the loudspeaker 724 will provide thepatient with emergency contact with a remote diagnostic center 708. Sucha system could work much like the ONSTAR emergency assistance system nowbuilt into many cars. For example, when a major or EMERGENCY alarm isidentified by the guardian implant 505, the following steps could befollowed:

[0290] 1. The guardian will first ascertain if an external alarmtransceiver is within range, if not the internal alarm will beinitiated.

[0291] 2. If the external alarm transceiver is within range the systemwill next see if there is access to the remote diagnostic center 708through the long distance voice/data communication interface 729. If notthe external alarm transceiver 720 and implant 505 will initiateinternal and/or external alarm notification of the patient.

[0292] 3. If there is access to the remote diagnostic center 708 thelong distance voice/data communication interface 729, the patient alarminformation including alarm and baseline electrogram segments will betransmitted to the remote diagnostic center 708. A medical professionalat the remote diagnostic center 708 will view the data and immediatelyestablish voice communication to the external alarm transceiver 720through the long distance voice/data communication interface 729. Ifthis occurs, the first thing that the patient will hear is a ringingtone and/or a voice announcement followed by the contact with themedical professional who can address the patient by name and facilitateappropriate emergency care for the patient. In this case, the internaland external alarms will not be needed and to the patient it willresemble an incoming telephone call from the medical professional. It isalso envisioned that the voice of the medical professional could be thefirst thing that the patient hears although an initial alerting signalis preferred.

[0293] This method of establishing the highest level of communicationavailable to the guardian system with the fall back of just the internalalarm will provide the best possible patient alerting based on what isavailable at the time of the alarm.

[0294] The data communications between the external alarm transceiver720 and the remote diagnostic center 708 would utilize a standardized(or custom) data communications protocol. For example, the datacommunications might utilize any or all of the following either within aprivate network, a VPN, an intranet (e.g. a single provider network suchas the Sprint data network) or through the public internet:

[0295] 1. Basic TCP/IP messaging within a single network or through theinternet.

[0296] 2. Short Messaging Service (SMS)

[0297] 3. Multimedia Message Service (MMS) used for cell phonetransmission

[0298] 4. Universal Datagram Protocol (UDP)

[0299] It is also envisioned that the present invention would takeadvantage of existing telephone network call center technology includinguse of Automatic Number Identification (ANI) to identify the incomingcall, and Dialed Number Identification Service (DNIS) where differentnumbers might be dialed by the external alarm transceiver 720 dependingon the severity of the detected cardiac event. For example, in the casewhere the call is placed by the emergency alarm transceiver 720, anEMERGENCY alarm might dial a different number than a SEE DOCTOR alertwhich might be different from a patient-initiated “panic button” call.DNIS could help get the appropriate help for the patient even if dataconnectivity is unavailable and might be used to prioritize which callis answered first (e.g., an EMERGENCY alarm would have higher prioritythan a SEE DOCTOR alert).

[0300] It is also envisioned that the remote diagnostic center 708 couldfacilitate the scheduling of an appointment with the patient's doctorfollowing a SEE DOCTOR alert.

[0301] Although throughout this specification all patients have beenreferred to in the masculine gender, it is of course understood thatpatients could be male or female. Furthermore, although the onlyelectrogram indications for an acute myocardial infarction that arediscussed herein are shifts involving the ST segment and T wave height,it should be understood that other changes in the electrogram (dependingon where in the heart the occlusion has occurred and where theelectrodes are placed) could also be used to determine that an acutemyocardial infarction is occurring. Furthermore, sensors such as heartmotion sensors, or devices to measure pressure, pO₂ or any otherindication of an acute myocardial infarction or cardiac events could beused independently or in conjunction with a ST segment or T wave shiftdetectors to sense a cardiac event.

[0302] It is also envisioned that all of the processing techniquesdescribed herein for an implantable cardiosaver are applicable to aguardian system configuration using skin surface electrodes and anon-implanted cardiosaver 5 the term electrogram would be replaced bythe term electrocardiogram. Thus the cardiosaver device described inFIGS. 5 through 12 would also function as a monitoring device that iscompletely external to the patient.

[0303] Various other modifications, adaptations, and alternative designsare of course possible in light of the above teachings. Therefore, itshould be understood at this time that, within the scope of the appendedclaims, the invention can be practiced otherwise than as specificallydescribed herein.

What is claimed is:
 1. A system for detecting a cardiac event in a human patient the system including: at least two implanted electrodes for sensing electrical signals from the patient's heart; an implantable device for detecting a cardiac event, the device including; means for periodically processing an electrogram segment of the electrical signals from the patient's heart, the processing being designed to detect an abnormality in the electrogram segment, the abnormality being the electrogram segment being processed for a predetermined segment time period, the segment time period occurring after a sleep state time period during which sleep state time period the implantable device does not process the electrical signals from the patient's heart; and means to reduce the sleep state time period when an abnormality is detected and means to increase the sleep state time period when no abnormality is detected.
 2. The system of claim 1 where the abnormality is a voltage of the ST segment whose magnitude exceeds a pre-set threshold.
 3. The system of claim 2 where the abnormality is indicative of acute myocardial infarction.
 4. The system of claim 2 where the abnormality is indicative of exercise induced ischemia.
 5. The system of claim 1 where the abnormality is an excessive ST shift.
 6. The system of claim 1 where the abnormality is indicative of an arrhythmia.
 7. The system of claim 6 where the arrhythmia is selected from the group consisting of tachycardia, bradycardia, unsteady heart rate, bigeminal rhythm, premature ventricular contractions, premature atrial contractions and atrial fibrillation.
 8. The system of claim 1 where at least one of the electrodes is located within the heart.
 9. The system of claim 8 where the electrode located within the heart is located within the right ventricle.
 10. The system of claim 1 where at least one electrode is located subcutaneously.
 11. The system of claim 1 where the sleep state time period is increased to a maximum sleep state time period if no abnormality is detected.
 12. The system of claim 1 where the sleep state time period is reduced to minimum sleep state time period if an abnormality is detected.
 13. The system of claim 1 where the sleep state time period is reduced to less than 60 seconds when an abnormality is detected.
 14. The system of claim 1 where the sleep state time period is increased to a maximum sleep time period if two or more successive electrogram segments have no detected abnormality.
 15. The system of claim 1 where the criteria for detecting an abnormality in an electrogram segment differs from the criteria to detect a cardiac event.
 16. A system for detecting a cardiac event in a human patient the system including: at least two implanted electrodes for sensing the electrogram signal from the patient's heart; an implanted device designed to store the electrogram signal from the patient's heart, the implanted device being further designed to detect an ischemic event when the magnitude of a voltage change of the ST segment of the electrogram signal exceeds a detection threshold; and means to process the stored electrogram signal to calculate the detection threshold.
 17. The system of claim 16 further including a programmer, the programmer providing the means to process the stored electrogram signal to calculate the detection threshold.
 18. The system of claim 17 where the detection threshold is set to be the mean value of the ST segment voltage plus a multiple of the standard deviation of the ST segment voltage for all the stored electrograms beats.
 19. The system of claim 16 where the detection threshold is calculated for each of at least two different heart rate ranges.
 20. The system of claim 16 further including patient alerting means.
 21. The system of claim 20 where the patient alerting means includes at least two types of alarm signals.
 22. The system of claim 21 where the patient alerting means is provided by the implanted device.
 23. The system of claim 22 where the patient alerting means is provided by an external alarm system.
 24. The system of claim 16 where the ischemic event is an acute myocardial infarction.
 25. The system of claim 24 where the ischemic event is ST elevation acute myocardial infarction.
 26. The system of claim 24 where the ischemic event is non-ST elevation acute myocardial infarction.
 27. The system of claim 16 where the ischemic event is an ST shift at an elevated heart rate.
 28. The system of claim 16 where the ischemic event is ST depression at an elevated heart rate.
 29. A method for setting the thresholds for detecting excessive shift in the voltage of the ST segment of a patient's electrogram, the method including the steps: (a) implanting a cardiosaver device having the capability to sense and store electrogram signals from the patient's heart; (b) causing the patient's heart to have at least two different heart rates where a first heart rate is a lower rate compared to a second heart rate which is a higher heart rate; (c) measuring the patient's ST segment voltage at each of the at least two different heart rates; (d) programming the detection thresholds for excessive shift in the voltage of the ST segment of the patient's electrogram based on the electrogram data processed for the at least two different heart rate ranges.
 30. The method of claim 29 where the means for causing the patient's heart rate to increase to the second, higher heart rate is a stress test.
 31. The method of claim 30 where the stress test is an exercise stress test.
 32. The method of claim 30 where the stress test is a drug induced stress test where medications are used to increase the patient's heart rate.
 33. The method of claim 29 where the means for causing the patient's heart rate to increase to the second, higher heart rate is to have the patient go about his normal daily activities and to record electrogram data during a period of increased physical activity which results in the second, higher heart rate.
 34. A system for identifying an ischemic event in a human patient including: at least two electrodes implanted in the patient for obtaining an electrical signal from the patient's heart, the electrical signal being an electrogram; an implanted cardiosaver including: (a) analog-to-digital converter circuitry for digitizing the electrogram to produce a multiplicity of electrogram segments, each having a time period of least 1 second duration; (b) memory means designed to store a baseline electrogram segment at a first predetermined time; (c) memory means designed to store a recently collected electrogram segment at a second time that is later than the first predetermined time; (d) processor means coupled to said memory means for comparing the ST segment voltage of at least one beat of the recently collected electrogram segment at the second time with the average ST segment voltage of at least two beats of the baseline electrogram segment stored in the memory means at the first predetermined time; and, (e) means for identifying that the ischemic event has occurred when change in the ST segment voltage of the at least one beat of the recently collected electrogram segment as compared to the average ST segment voltage of the at least two beats of the baseline electrogram segment exceeds a detection threshold, the detection threshold being a preset percentage of the average signal amplitude of the at least two beats of the baseline electrogram segment.
 35. The system of claim 34 where the average signal amplitude of the at least two beats of the baseline electrogram segment is the R wave peak voltage minus the average PQ segment voltage averaged over the at least two beats of the baseline electrogram.
 36. The system of claim 34 where the average signal amplitude of the at least two beats of the baseline electrogram segment is the average peak-to-peak, R-to-S, signal amplitude of the at least two beats of the baseline electrogram.
 37. The system of claim 34 where the average signal amplitude of the at least two beats of the baseline electrogram segment is the difference between the average PQ segment voltage and the peak S wave voltage averaged over the at least two beats of the baseline electrogram segment.
 38. The system of claim 34 where the average signal amplitude of the at least two beats of the baseline electrogram segment is the larger of the R wave to PQ segment voltage difference or the PQ segment to S wave voltage difference.
 39. A system for detecting a cardiac event in a human patient, the system including: at least two electrodes implanted in the patient for obtaining the electrical signal from the patient's heart, the electrical signal being an electrogram; an implanted cardiosaver having patient alerting means, the cardiosaver designed to detect a cardiac event by processing the electrical signal from the patient's heart, the cardiosaver designed to categorize detected cardiac events into at least two different levels of severity;
 40. The system of claim 39 further including: a remote diagnostic center designed to receive data messages and to send and receive voice telephone calls, the remote diagnostic center also designed to prioritize calls based on the level of severity of the detected cardiac event; and an external alarm transceiver having two-way data communications with the cardiosaver, the two-way data communications including the ability of the external alarm transceiver to receive notification of a cardiac event detected by the cardiosaver, the notification including the level of severity of the cardiac event, the external alarm transceiver also having the capability for data communication with the remote diagnostic center, the data communication including data messages indicating the level of severity of the detected cardiac event.
 41. The system of claim 39 where there are exactly two levels of severity for detected cardiac events.
 42. The system of claim 39 where one level of severity is an EMERGENCY ALARM indicating that the patient should seek immediate medical attention.
 43. The system of claim 39 where one level of severity is a SEE DOCTOR ALERT indicating that the patient should schedule an appointment with a medical practitioner as soon as convenient.
 44. A system for detecting a cardiac event in a human patient, the system including: at least two skin surface electrodes for obtaining the electrical signal from the patient's heart, the electrical signal being an electrocardiogram; an externally located cardiosaver having patient alerting means, the cardiosaver designed to detect a cardiac event by processing the electrical signal from the patient's heart, the cardiosaver designed to categorize detected cardiac events into at least two levels of severity.
 45. The system of claim 44 further including: a remote diagnostic center designed to receive data messages and to send and receive voice telephone calls, the remote diagnostic center also designed to prioritize calls based on the level of severity of the detected cardiac event; and an external alarm transceiver having two-way data communications with the cardiosaver, the two-way data communications including the ability of the external alarm transceiver to receive notification of a cardiac event detected by the cardiosaver, the notification including the level of severity of the cardiac event, the external alarm transceiver also having the capability for data communication with the remote diagnostic center, the data communication including data messages indicating the level of severity of the detected cardiac event.
 46. The system of claim 45 where the external alarm transceiver is incorporated into the external cardiosaver.
 47. The system of claim 44 where there are exactly two levels of severity for detected cardiac events.
 48. The system of claim 44 where one level of severity is an EMERGENCY ALARM indicating that the patient should seek immediate medical attention.
 49. The system of claim 44 where one level of severity is a SEE DOCTOR ALERT indicating that the patient should schedule an appointment with a medical practitioner as soon as convenient.
 50. An implantable system for detecting cardiac events in a human patient, the system including: at least two electrodes implanted in the patient for obtaining the electrical signal from the patient's heart, the electrical signal being an electrogram; patient alerting means; and a primary battery and a secondary battery where the primary battery has a larger capacity than the secondary battery.
 51. The system of claim 50 where the secondary battery operates at a higher voltage than the primary battery.
 52. The system of claim 50 where the secondary battery can provide a higher maximum current than the primary battery.
 53. The system of claim 50 where the secondary battery is rechargeable.
 54. The system of claim 53 where the secondary battery is recharged by the primary battery through a charging circuit.
 55. The system of claim 50 where the secondary battery provides power to the patient alerting means.
 56. The system of claim 50 further including a telemetry sub-system where the secondary battery provides power to the telemetry sub-system.
 57. The system of claim 50 where the primary battery has a lower self discharge rate than the secondary battery.
 58. The system of claim 50 further including pacemaker circuitry.
 59. The system of claim 50 further including cardiac defibrillator circuitry.
 60. The system of claim 50 where the cardiac events detected by the system include acute myocardial infarction.
 61. The system of claim 50 where the cardiac events detected by the system include ischemia at an elevated heart rate.
 62. The system of claim 50 where the cardiac events detected by the system include low heart rate.
 63. The system of claim 50 where the cardiac events detected by the system include excessively high heart rate.
 64. The system of claim 50 where the cardiac events detected by the system include arrhythmias.
 65. A system for detecting cardiac events in a human patient, the system including: at least two electrodes for obtaining an electrical signal from the patient's heart; processor means designed to detect at least one type of cardiac event by processing the electrical signal from the patient's heart; patient alerting means that are activated following the detection of a cardiac event by the processor means, the patient alerting means providing an alarm signal to the patient for a first period of time which is an initial alarm on-period, the initial alarm on-period being followed by an alarm off-period of time during which time the alarm signal is turned off, the patient alerting means also providing a reminder alarm signal to the patient for a reminder alarm on-period of time following the alarm off-period of time.
 66. The system of claim 65 where reminder alarm signal which lasts for the reminder alarm on-period is repeated periodically, the time between the periodic reminder alarm signals being an additional alarm off-period.
 67. The system of claim 66 where the periodic reminder alarm on-period is greater than 15 minutes.
 68. The system of claim 66 where the periodic reminder alarm on-period is less than 5 hours.
 69. The system of claim 66 where the alarm off-period after the initial alarm on-period has a different time duration as compared to the time duration of the alarm off-period after a periodic reminder alarm on-period.
 70. The system of claim 65 where the alarm off-period of time is at least twice as long as the alarm on-period.
 71. The system of claim 65 further including an external alarm system having an alarm disable button.
 72. The system of claim 65 where the alarm on-period is less than 10 minutes.
 73. The system of claim 65 where at least a portion of the system is implanted in the human patient.
 74. The system of claim 65 where the cardiac events detected by the system include acute myocardial infarction.
 75. The system of claim 65 where the cardiac events detected by the system include ischemia at an elevated heart rate.
 76. The system of claim 65 where the cardiac events detected by the system include low heart rate.
 77. The system of claim 65 where the cardiac events detected by the system include excessively high heart rate.
 78. The system of claim 65 where the cardiac events detected by the system include arrhythmias.
 79. A system for detecting cardiac events in a human patient, the system including: at least two electrodes for obtaining the electrical signal from the patient's heart; patient activity monitoring means including an accelerometer designed to identify when the patient is active; and processor means designed to process the electrical signal from the patient's heart to detect at least one type of cardiac event that occurs when the patient activity monitoring means indicates that the patient is active.
 80. The system of claim 79 where the entire system is implanted into the patient.
 81. The system of claim 79 further including patient alerting means that are activated following the detection of a cardiac event by the processor means.
 82. The system of claim 79 where the cardiac event detected is exercise induced ischemia.
 83. The system of claim 79 where the cardiac event is an arrhythmia.
 84. The system of claim 79 further including means to record the electrical signal from the patient's heart.
 85. A system for displaying segments of an electrogram of a patient in whom a cardiac event has previously been detected, the system including at least two electrodes implanted in the patient for obtaining the electrical signal from the patient's heart which electrical signal is the electrogram, the system also including an implanted cardiosaver and an external alarm system; the implanted cardiosaver including: (a) analog-to-digital converter circuitry for digitizing the electrogram to produce a baseline electrogram segment at a first predetermined time and a recently collected electrogram segment at a second predetermined time that is later than the first predetermined time; each electrogram segment having a time duration that is at least 1 second; (b) memory means designed to store the baseline electrogram segment and the recently collected electrogram segment (c) processor means coupled to said memory means to detect a cardiac event by comparing the recently collected electrogram segment with the baseline electrogram segment, and the external alarm system having two-way wireless communication to and from the implanted cardiosaver, the external alarm system also having an external alarm signal that is actuated when the implanted device detects the cardiac event, the external alarm system also having means to display the baseline and recently collected electrogram segments whose comparison resulted in the detection of the cardiac event.
 86. A system for detecting ischemia in a human patient, the system including at least two electrodes implanted in the patient for obtaining the electrical signal from the patient's heart; and an implanted cardiosaver designed to process the electrical signal to measure the ST segment voltage and to identify R waves, the processing of the electrical signal designed to identify the R waves including a separate high pass filter having greater low frequency attenuation as compared with the high pass filter of the electrical signal used for measurement the ST segment voltage.
 87. The system of claim 86 further including implantable cardiac defibrillator circuitry.
 88. The system of claim 86 further including pacemaker circuitry. 